Acceleration sensor

ABSTRACT

An acceleration sensor comprises a fixed case member and a cover assembly collectively defining a closed space in which the oscillation plate and the piezoelectric element received therein. The oscillation plate and the piezoelectric element are oscillatably supported by a supporting portion formed on the central bottom portion of the fixed case member. The oscillation plate and the piezoelectric element are integrally oscillatable in two different modes consisting of: a {fraction (1/1)} oscillation mode where the oscillation plate is irregularly deformed to have the peripheral portion oscillated with a single vector in the oscillation direction of the oscillation plate when the oscillation plate is oscillated with respect to the fixed case member at a resonance frequency f 0 ; and a ½ oscillation mode where the oscillation plate is irregularly deformed to have two different half parts of the peripheral portion oscillated with their respective different vectors opposite to each other in the oscillation direction of the oscillation plate when the oscillation plate is oscillated with respect to the fixed case member at a noise frequency f 0   1 , and the resonance frequency f 0  and the noise frequency f 0   1  are out of the range of effective oscillation frequencies. Thus constructed acceleration sensor is of high performance and appropriate for automatic production at a low cost.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acceleration sensor, and moreparticularly to an acceleration sensor for detecting an acceleration bytransforming oscillation levels into electrical signals.

2. Description of the Related Art

In general, the acceleration sensor now in use includes various typessuch as an electro magnetic type, a piezoelectric element type, and asemiconductor type which are known as detecting an acceleration appliedthereto. Among these types of acceleration sensor, the piezoelectricelement type of the acceleration sensor has a piezoelectric elementdeformable in response to the acceleration to detect the acceleration.These piezoelectric element types of the acceleration sensor are appliedto automotive vehicles and used for controlling knocking of engine andair bag.

A conventional piezoelectric element type of the acceleration sensorherein raised for example is shown in FIG. 25 to comprise an oscillationplate having a central portion fixed. This type is called “thecenter-fixed type of acceleration sensor”, i.e., the first conventionalacceleration sensor. This center-fixed type of acceleration sensor 100comprises a fixed metal case 101 having a central bottom portion fromwhich projects a supporting protrusion 101 a integrally formed with thecentral portion. Onto the supporting protrusion 101 a is welded andsecurely connected an oscillation plate 102 made of a metal and in theform of a thin disc shape to facilitate resonance motion of theoscillation plate 102 as shown in FIG. 26. On the upper surface of theoscillation plate 102 is mounted a piezoelectric element 103 in adoughnut shape in a manner that the piezoelectric element 103 is held inaxial alignment with the oscillation plate 102. The piezoelectricelement 103 has upper and lower surfaces on which are respectivelymounted a pair of electrodes 104 axially aligned with the piezoelectricelement 103. One of the electrodes 104 is electrically connected withthe oscillation plate 102, while the other one of the electrodes 104 issoldered at 105 a and thus electrically connected with a metal wire 105by way of, for instance, wire bonding. The acceleration sensor 100further comprises an output terminal 107 having one end electricallyconnected with the metal wire 105 and the other end electricallyconnected with an exterior connector, not shown, and a cover member 106in the form of a bowl shape and made of a resin material. The fixed case101 and the cover member 106 have peripheral edge portions 101 c and 106c, respectively, which are firmly coupled with each other to define aclosed space 109 having the oscillation plate 102 and the piezoelectricelement 103 received therein. Between the peripheral edge portions 101 cof the fixed case 101 and 106 c of the cover member 106 is disposed anO-ring which serves to hermetically seal the closed space 109.

Another conventional piezoelectric element type of the accelerationsensor herein raised for example, i.e., the second conventionalacceleration sensor is shown in FIG. 27. The acceleration sensor 110comprises a fixed case 111 made of a metal and having a peripheral ledgeportion 111 c, and a metal base member 112 in the form of a disc shapeand also having a peripheral edge portion 112 c. The metal base member112 is welded to and thus securely mounted on the fixed case 111 withthe peripheral ledge portion 111 c being in registry with the peripheraledge portion 112 c so that the fixed case 111 is covered and closed bythe metal base member 112. On the metal base member 112 is mounted aconnector member 116 also in the form of a disc shape and having aperipheral edge portion 116 c fixedly engaged with the peripheral ledgeportion 111 c of the fixed case 111. The connector member 116 has anoutput terminal 107 securely mounted thereon and is electricallyconnected with an exterior connector, not shown. The fixed case 111, themetal base member 112 and the connector member 116 collectively define aclosed space 109 in which the oscillation plate 102 and thepiezoelectric element 103 are accommodated. The metal base member 112has a central portion from which downwardly extends a protrusion 112 ahaving the oscillation plate 102 supported thereon, compared with theprotrusion 101 a of the case base 101 upwardly projected and having theoscillation plate 102 supported thereon as shown in FIG. 25. Both of theoscillation plate 102 and the piezoelectric element 103 are in the formof a doughnut shape and securely supported by the protrusion 112 a ofthe metal base member 112 to ensure that the oscillation plate 102 isoscillatable with respect to the fixed case 111. The connector member116 is made of a resin material and serves to electrically insulate themetal base member 112 from the fixed case 111. The output terminal 107securely mounted on the connector member 116 extends through theprotrusion 112 a of the metal base member 112 and has a lower endelectrically connected with one of the electrodes 104 on thepiezoelectric element 103 by way of a connecting disc plate 115 solderedat 115 a to the lower end of the output terminal 107 and one of theelectrode 104. The acceleration sensor 110 comprises an O-ring 118disposed between the inner peripheral face of the fixed case 111 and theouter peripheral face of the metal base member 112 to hermetically sealthe closed space 109. It is preferable that the connecting disc plate115 has a rigidity as small as possible so that the oscillation plate102 and the piezoelectric element 103 are not prevented from beingoscillated. The connecting disc plate 115 may be replaced with a metalwire having one end electrically connected to the output terminal 107and the other end electrically connected to one of the electrode 104 onthe piezoelectric element 103 in a manner that the oscillation plate 102is welded on the protrusion 112 a of the metal base member 112.

The first and second conventional acceleration sensors 100 and 110respectively have lower portions formed with male screws 101 b and 111 beach screwed in to an oscillation object such as an automotive engine orthe like to ensure that the oscillation plate 102 is oscillated withrespect to the fixed cases 101 and 111 when the oscillation object isoscillated for some reason. The oscillation of the oscillation plate 102causes the piezoelectric elements 103 to be deformed and energized togenerate voltage levels which are outputted to the output terminal 107through one of the electrodes 104 with the fixed case 101 or 111 and themetal base member 112 grounded.

In general, the piezoelectric element 103 has a capacity C between theelectrodes 104 which can produce an electric charge Q when theoscillation plate 102 is oscillated and deformed to produce a stressdeformation in the piezoelectric element 103 by exterior oscillations,i.e. accelerations. The electric charge Q thus caused by the stressdeformation of the oscillation plate 102 can be measured as voltage Vthat is represented by the following equation:

V=Q/C

It is considered that the oscillation plate 102 has the maximumoscillation amplitude at around its outer peripheral end while thepiezoelectric element 103 has the maximum stress deformation value ataround its central portion, resulting from the fact that thepiezoelectric element 103 is extended and contracted.

The acceleration sensor 100 or 110 has a frequency characteristic undera predetermined level of oscillation corresponding to a predeterminedlevel of acceleration as shown in FIG. 28. FIG. 28 indicates that theoutput voltage V₀ is high at a frequency of the resonance point f₀,hereinlater referred to as “resonance frequency f₀”, while being flatand low at frequency points in other areas such as medium and lowfrequency areas. In view of this frequency characteristic, accelerationsensors such as the acceleration sensors 100 and 110 are classified intotwo different types consisting of a non-resonance type of using a flatportion of the frequency characteristics within a predetermined range ofeffective frequencies which does not include the resonance frequency f₀and a resonance type of using frequency characteristics having theresonance frequency f₀ within a predetermined range of effectivefrequencies. The acceleration sensors 100 and 110 are adapted to operatewith the oscillation plate 102 oscillated at a desired frequency levelwithin a predetermined range of effective frequencies having the upperlimit in the vicinity of the resonance frequency f₀.

The resonance frequency f₀ of the oscillation plate 102 in the form of adisc shape and securely mounted on the central portion of the fixed casecan be represented by the following equation (1).

[Eq. 1]

f ₀=α(t/R ²)×{square root over (E/ρ(1−σ²))}  equation (1)

where α=0.172 (constant), t stands for thickness, R stands for radius, Estands for Young's modulus, ρ stands for density, and σ stands forPoisson's ratio.

In the event that the oscillation plate 102 is made of nickel steel, theabove parameters are as follows.

t=0.4 (mm),

R=7 (mm),

E=2×10¹¹ (N/m ²),

ρ=7.8×10³ (kg/m³),

and

σ=0.28.

The above parameters render the resonance frequency f₀ to be 7.41 (kHz).The resonance frequency f₀ is determined primarily by the oscillationplate 102, however, should be decided in consideration of otherneighboring elements such as the fixed case 101 and piezoelectricelement 103. This is because of the fact that those elements slightlyaffect the oscillation of the oscillation plate 102.

In order to secure a desired resonance frequency f₀ in view of the abovefact, the thickness t and the radius R are, in general, required to beappropriately selected for designing the acceleration sensor. Inparticular, the resonance frequency f₀ is affected largely by the radiusR as will be seen from the fact based on experimental results that theresonance frequency f₀ is varied by a rate of about 1 to 2% as theradius R of the oscillation plate 102 is varied by 0.1 mm with thethickness t unchanged. In the light of the sensitivity of theacceleration sensor, it is evident through repeated experiments that theacceleration sensor 110 shown in FIG. 27 can be produced withsensitivity higher than that of the acceleration sensor 100 shown inFIG. 25. The reason is considered to be due to the fact that theoscillation plate 102 is mounted on the metal base member 112, with theresult that the metal base member 112 being not completely rigid isslightly oscillated together with the oscillation plate 102 when itreceives acceleration, thereby making it possible for the oscillation ofthe oscillation plate 102 to be amplified by the metal base member 112.

The electrodes 104 mounted on the piezoelectric element 103 may becategorized into two different groups consisting of a first group ofexcitation electrodes which is constituted by a pair of electrodes witha small diameter and a second group of detection electrodes which isconstituted by a pair of electrodes with a large diameter, and both thefirst group of the exciting electrodes and the second group of thedetection electrodes are coaxially aligned with the piezoelectricelement 103. Alternating current is applied to the piezoelectric element103 through the excitation electrodes thus constructed so as tooscillate the oscillation plate 102 by way of the piezoelectric effect,and energize the detection electrodes, thereby making it possible tomeasure output voltage through the detection electrodes for carrying outthe self diagnostics such as performance and failure diagnostics, or thecalibration of the acceleration sensor. In the conventional accelerationsensors 100 and 110, the oscillation plate 102 is supported by thesupporting protrusion 101 a and the protrusion 112 a, respectively.There are, however, provided many variations of the acceleration sensor.The oscillation plate may be in the form of a disc shape having aperipheral portion clamped, or in the form of a rod having one endsecurely mounted. The fixed cases 101 and 111 are classified into twotypes consisting of one-terminal type of having the fixed case serve asa ground and two-terminal type having two terminals, one of which servesas a ground.

FIG. 29 shows a third conventional acceleration sensor 120 of thepiezoelectric element type and the non-resonance type comprising apiezoelectric element and a weight. This type is called “the compressiontype of the acceleration sensor”. The acceleration sensor 120 comprisesa connector body 126 and a fixed case 121. The connector body 126 has aperipheral edge portion. The fixed case 121 is made of a metal materialand has an open peripheral end portion 121 c which is bent to form afitting portion fittingly connected with the peripheral edge portion ofthe connector body 126 to define a closed space 109 having a weight 122and a piezoelectric element 123 received therein. The connector body 126has a terminal 107 mounted thereon. The piezoelectric element 123 is inthe form of a doughnut shape and has upper and lower surfaces on whichare respectively mounted a pair of detection electrodes 124 consistingof a first electrode and a second electrode 124 a and 124 b. The weight122 is made of a metal material and in the form of a cylindrical shape.The weight 122 is held in contact with the first detection electrodes124 a on the upper surface of the piezoelectric element 123 as shown inFIG. 30. The terminal 107 is adapted to be electrically connected to thefirst electrode 124 a of the piezoelectric element 123 and an exteriorconnector, not shown. The weight 122 is securely mounted on thepiezoelectric element 123 by means of a fastening screw 125 topressurize the piezoelectric element 123 toward the center bottomportion of the fixed case member 121. The fastening screw 125 is screwedin through a screw hole 121 d formed in the center bottom portion of thefixed case 121.

The second detection electrodes 124 b forming part of the accelerationsensor 120 is mounted on the lower surface of the piezoelectric element123 to be electrically connected with the fixed case 121 while the firstdetection electrodes 124 a is mounted on the upper surface of thepiezoelectric element 123 to be electrically connected with the weight122 and a contact terminal 127. The contact terminal 127 is in the formof a L-shape and securely mounted on the weight 122 by the fasteningscrew 125. The contact terminal 127 is electrically connected with theoutput terminal 107 of the connector body 126 through a wire 129 havingboth ends 129 a and 129 b soldered with the contact terminal 127 and theoutput terminal 107, respectively. The acceleration sensor 120 furthercomprises an insulation tube 125 a and an insulation spacer 125 binterposed between the weight 122, the piezoelectric element 123, andthe fastening screw 125 to prevent the fixed case 121 and the outputterminal 107 from forming a short circuit. The acceleration sensor 120further comprises an O-ring 128 disposed between the open peripheral endportion 121 c of the fixed case 121 and the peripheral end portion ofthe connector body 126 to hermetically seal the closed space 109 inwhich electrical components such as the piezoelectric element 123 areaccommodated.

The acceleration sensor 120 thus constructed makes it possible to usethe fixed case 121 as a ground for an electric circuit, and output anoutput voltage of the piezoelectric element 123 through the weight 122and the output terminal 107. The fixed case 121 has a bottom portionformed with a male screw 121 b fixed to an exterior object such as anengine, not shown, to be detected for an acceleration. An oscillationcaused by the exterior object is transmitted to the weight 122, whichexerts a load (compression force) on the piezoelectric element 123 inresponse. The piezoelectric element 123 generates an output voltageindicative of the acceleration and outputs the output voltage throughthe output terminal 107. The acceleration is thus detected on the basisof the output voltage received from the output terminal 107. Theacceleration sensor 120 has a frequency characteristic similar to thatof the aforesaid acceleration sensors 100 and 110 under a predeterminedconstant level of oscillation, i.e., constant acceleration as shown inFIG. 28. The resonance frequency f₀, however, does not appear to arecognizable extent depending upon the condition of the accelerationsensor assembled with other devices and machines. This results from thefact that the resonance frequency f₀ moves to a higher frequency rangedue to the fact the fastening screw 125 is screwed in through thecentral portion of the piezoelectric element 123 and the weight 122 witha relatively small screwing force exerted on the peripheral portion ofthe acceleration sensor 120, thereby causing the acceleration sensor 120to be resonantly oscillated in a high frequency range. This means thatthe fastening screw 125 is required to be produced with high precisionsfor torque and machining of the engagement faces of the fastening screw125.

The acceleration sensor 120 of such non-resonance frequency type isusually designed to be oscillatable with the resonance frequency f₀ of20 kHz or greater, which is out of the range of effective oscillationfrequencies, so that the flat portion, i.e., V₀ of the output voltagerange is actually used for detecting an acceleration (see FIG. 28). V₀also stands for “the sensitivity” of the acceleration sensor. The basicprinciple of the acceleration sensor 120 is that an acceleration [G]exerted on a weight 122 of mass [m] causes a stress strain [F] on thepiezoelectric element 123 to generate an output voltage V₀ indicative ofthe acceleration in accordance with the equation as follows.

F=m·G

 V ₀ ≈α·F·t/S

where α stands for a constant such as piezoelectric constant, S standsfor the area of the detecting electrode 124 of the piezoelectric element123, and t stands for the thickness of the piezoelectric element 123.

As will be understood from the foregoing description, the methods toenhance the sensitivity of the acceleration sensor 120 is considered toinclude:

(1) an increased weight of the weight 122, and/or

(2) an increased factor “t/S” of the piezoelectric element 123. (Theincrease in the factor “t/S”, however, is limited to a predeterminedlevel decided based on its size and volume requested.)

It is therefore understood that the size, especially, the height of theacceleration sensor is required to be enlarged in order to enhance thesensitivity.

The acceleration sensor 120 may comprise a gold plated connectingterminal in place of a lead line such as the wire 129 having the outputterminal 107 electrically connected with the weight 122 (the contactterminal 127). The acceleration sensor 120 is not limited to theone-terminal type of having the fixed case 121 serve as a ground butalso includes the two-terminal type having two terminals, one of whichserves as a ground. The electrodes 124 a and 124 b of the piezoelectricelement 123 may be divided into two groups consisting of the first groupof electrodes serving for detecting an acceleration and the second groupof electrodes serving for performing the self diagnostics orcalibration.

As will be seen from the forgoing description, the first conventionalacceleration sensor 100, however, encounters such problems that it isdifficult to automatically assemble the acceleration sensor 100resulting from the fact that one of the electrodes 104 of thepiezoelectric element 103 is required to be electrically connected withthe output terminal 107 of the cover member 106 through the wire 105having both ends soldered with them, respectively, by way of, forinstance, wire bonding. This leads to the fact that the production costof the acceleration sensor 100 rises.

As will be seen from the foregoing description, the second conventionalacceleration sensor 110 requires no wire connections, thus makes itpossible to automatically assemble the acceleration sensor 110 andimprove the sensitivity in comparison with the first conventionalacceleration sensor 100. The second conventional acceleration sensor110, however, encounters another problem that oscillation in a highfrequency range beyond 10 kHz is easily transmitted throughconstitutional parts and elements within the acceleration sensor 110such as the fixed case 111, and the oscillation thus transmitted affectsthe characteristics of the acceleration sensor 110 such as phasecharacteristics. The second conventional acceleration sensor 110 alsoencounters another problem that the metal base member 112 is notperfectly rigid but could be slightly distorted and loosened due totemperature degradation resulting from the fact that the connectormember 116 has a peripheral edge portion fixedly engaged with theperipheral ledge portion 111 c of the fixed case 111, and a gap betweenthe fixed case 111, the metal base member 112, and the connector member116 is subject to vary at an elevated temperature. An oscillation noisegenerated from the output terminal 107 is transmitted to the connectormember 116. The metal base member 112 thus distorted and loosened willtransmit the oscillation noise to the oscillation plate 102, therebydeteriorating the accuracy of the acceleration sensor 110 for detectingan acceleration.

Furthermore, the first and second acceleration sensors 100 and 110encounter another problem. As a result of an analysis by means of thefinite element method, the oscillation plate of acceleration sensors ofthe center-fixed type such as the acceleration sensors 100 and 110 isoscillatable in three different modes consisting of a {fraction (1/1)}oscillation mode, a ½ oscillation mode, and a ¼ oscillation mode asshown in FIG. 31.

FIG. 31A shows the oscillation plate oscillating in the {fraction (1/1)}oscillation mode where the oscillation plate is irregularly deformed tohave the peripheral portion oscillated with a single vector in theoscillation direction of the oscillation plate when the oscillationplate is oscillated with respect to the fixed case member at a resonancefrequency f₀, FIG. 31B shows the oscillating plate oscillating in the ½ocillation mode where the oscillation plate is irregularly deformed tohave two different half parts of the peripheral portion oscillated withtheir respective different vectors opposite to each other in theoscillation direction of the oscillation plate when the oscillationplate is oscillated with respect to the fixed case member at a firstnoise frequency f₀ 1, and FIG. 31C shows the oscillating plateoscillating in the ¼ oscillation mode where the oscillation plate isirregularly deformed to have four different parts of the peripheralportion oscillated with their respective different vectors opposite toone another in the oscillation direction of the oscillation plate whenthe oscillation plate is oscillated with respect to the fixed casemember at a second noise frequency f₀ 2. The first noise frequency f₀ 1is approximately half of the resonance frequency f₀, and the secondnoise frequency f₀ 2 is in the vicinity of the resonance frequency f₀.The oscillation of the oscillation plate in the ½ or ¼ ocillation modedoes not cause any problem as long as the oscillation plate has two orfour different parts of the peripheral portion evenly oscillated withrespective vectors opposite to one another in the oscillation directionof he oscillation plate, and the output voltage thus generated iscounterbalanced. The oscillation plate, however, could have two or fourdifferent parts of the peripheral portion unevenly oscillated withrespective vectors opposite. The uneven oscillation of the oscillationplate causes the piezoelectric element to generate a noise outputvoltage and deteriorate the accuracy of the acceleration sensor.Especially the oscillation of the oscillation plate in the ½ oscillationmode causes noise output voltage, hereinlater referred to as “spurious”.This leads to the fact that the oscillation of the oscillation plate ata frequency in the vicinity of the first noise frequency f₀ 1 causes anerror in detecting an acceleration.

The oscillation plate used for the acceleration sensor of thenon-resonance type is thick. It is considered that the weight balance ofthe oscillation plate with respect to the support portion affects thequality of the acceleration sensor.

As will be seen from the foregoing description, the third conventionalacceleration sensor 120 encounters a problem that it is difficult toautomatically assemble the acceleration sensor 120, and thus theproduction cost of the acceleration sensor 120 rises resulting from thefact that the acceleration sensor 120 has many parts and is complex inconstruction. The third conventional acceleration sensor 120 alsoencounters another problem that the acceleration sensor 120 is requiredto be produced with high precision for torque and machining of theengagement faces of the fastening screw 125 resulting from the fact thatthe fastening screw 125 is screwed in through the central portion of theweight 122 and the piezoelectric element 123 so that the weight 122 andthe piezoelectric element 123 are tightly held in contact with eachother toward the bottom surface of the fixed case 121. This furtherleads to another problem that the size (especially, the height) of theacceleration sensor 120 is required to be enlarged and the productioncost is increased. The acceleration sensor 120, furthermore, encountersanother problem that the fastening screw 125 may be loosened, therebycausing the acceleration sensor 120 to deteriorate the accuracy fordetecting an acceleration.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide anacceleration sensor which is simple in construction and thus constitutedby a small number of parts and elements.

It is another object of the present invention to provide an accelerationsensor which is most appropriate for automatic production ofacceleration sensors.

It is a further object of the present invention to provide anacceleration sensor which is inexpensive in production cost.

It is a yet further object of the present invention to provide anacceleration sensor which is excellent in performance.

In accordance with a first aspect of the present invention, there isprovided an acceleration sensor for detecting an acceleration caused byan object oscillated in an oscillation direction, comprising a sensorcasing having a center axis and to be positioned in coaxial alignmentwith the oscillation direction to receive the acceleration, the sensorcasing including a cylindrical fixed case member having a supportingportion axially extending, and a cover assembly provided on the fixedcase member to cover the fixed case member to define a closed space, anoscillation plate accommodated in the closed space of the sensor casingand having a central portion supported by the supporting portion of thefixed case member, and a peripheral portion integrally formed with thecentral portion and extending radially outwardly of the central portionto be freely movable with respect to the supporting portion of the fixedcase member, and the oscillation plate being partly oscillatable alongthe center axis with respect to the fixed case member, a piezoelectricelement held in contact with the oscillation plate to generate a voltageindicative of the acceleration when the acceleration is exerted on thesensor casing to have the oscillation plate partly oscillated along thecenter axis with respect to the sensor casing with the peripheralportion of the oscillation plate being deformed, and the piezoelectricelement having first and second surfaces respectively having first andsecond electrodes mounted thereon to have the voltage indicative of theacceleration to output therethrough; a terminal pin extending throughthe cover assembly and terminating at the exterior of the coverassembly, and a printed board retained by the cover assembly to have thesecond electrode of the piezoelectric element and the terminal pinconnected with each other.

In the above acceleration sensor, the fixed case member and theoscillation plate may be each made of an electroconductive material toensure that the first electrode is electrically connected with theoscillation plate and the fixed case member. The cover assemblycomprises a metal base member made of an electroconductive material anda cover member made of an insulating material. The metal base member hasa peripheral end portion welded to part of the fixed case member withthe closed space defined by the metal base member and the fixed casemember. The cover member is mounted on the metal base member with theprinted board interposed between the metal base member and the covermember, and the terminal pin extends through the metal base member, theprinted board, and the cover member and electrically connected with thesecond electrode of the piezoelectric element.

In accordance with a second aspect of the present invention, there isprovided an acceleration sensor for detecting an acceleration caused byan object oscillated in an oscillation direction, comprising: a sensorcasing having a center axis and to be positioned in coaxial alignmentwith the oscillation direction to receive the acceleration, the sensorcasing including a cylindrical fixed case member having a supportingportion axially extending, and a cover assembly provided on the fixedcase member to cover the fixed case member to define a closed space, anoscillation plate accommodated in the closed space of the sensor casingand having a central portion supported by the supporting portion of thefixed case member, and a peripheral portion integrally formed with thecentral portion and extending radially outwardly of the central portionto be freely movable with respect to the supporting portion of the fixedcase member, and the oscillation plate being partly oscillatable alongthe center axis with respect to the fixed case member, a piezoelectricelement held in contact with the oscillation plate to generate a voltageindicative of the acceleration when the acceleration is exerted on thesensor casing to have the oscillation plate partly oscillated along thecenter axis with respect to the sensor casing with the peripheralportion of the oscillation plate being deformed, and the piezoelectricelement having first and second surfaces respectively having first andsecond electrodes mounted thereon to have the voltage indicative of theacceleration to output therethrough; first and second terminal pinsextending through the cover assembly and terminating at the exterior ofthe cover assembly, and a printed board retained by the cover assemblyto have the electrodes of the piezoelectric element and the terminalpins connected with each other.

In the above acceleration sensor, the fixed case member and theoscillation plate may be each made of an electroconductive material toensure that the first electrode is electrically connected with theoscillation plate and the fixed case member. The cover assemblycomprises a metal base member made of an electroconductive material anda cover member made of an insulating material. The metal base member mayhave a peripheral end portion welded to part of the fixed case memberwith the closed space defined by the metal base member and the fixedcase member. The cover member is mounted on the metal base member withthe printed board interposed between the metal base member and the covermember. The first terminal pin extends through the metal base member,the printed board and the cover member to be electrically connected withthe first electrode of the piezoelectric element by way of the fixedcase member, the metal base member and the printed board. On the otherhand, the second terminal pin extends through the metal base member, theprinted board and the cover member to be electrically connected directlywith the second electrode of the piezoelectric element.

In the above acceleration sensor, the metal base member is formed with acentral hole, and the terminal pin having a contacting rod portionprojecting from the printed board and extending through the central holeof the metal base member to project into the closed space in electricalconnection with the second electrode of the piezoelectric element.

In the above acceleration sensor, the printed board may have a signalprocessing circuit.

In the above acceleration sensor, the cover member has a first surfacefirmly held in contact with the metal base member and a second surfaceopen to the atmosphere, and the fixed case member has a large diameterportion, a small diameter portion, and an annular ledge portion havingthe large and small diameter portions integrally formed with each otherto have the peripheral end portion of the metal base member mountedthereon and welded thereto. The small diameter portion has an openperipheral edge inwardly bent to be firmly engaged with the secondsurface of the cover member.

In the above acceleration sensor, the cover member is smaller indiameter than the metal base member to form an annular gap between theinner surface of the small diameter portion of the fixed case member andthe outer peripheral end surface of the cover member, and which furthercomprises a resilient ring disposed in the annular gap and rested on themetal base member to have the closed space hermetically sealed.

In the above acceleration sensor, the resilient ring may be made of anO-ring.

In the above acceleration sensor, the cover member may be formed with acircular recess having the printed board received therein, and anannular groove open to the circular recess. The metal base member isformed with an annular projection extending through the printed boardand snugly received in the annular groove of the cover member to havethe cover member positioned with respect to the metal base member withthe printed board positioned interposed between the cover member and themetal base member.

In the above acceleration sensor, the fixed case member may have a screwportion to be screwed to an exterior object which is to receive theacceleration.

In accordance with a third aspect of the present invention, there isprovided an acceleration sensor for detecting an acceleration caused byan object oscillated in an oscillation direction, comprising a sensorcasing having a center axis and to be positioned in coaxial alignmentwith the oscillation direction to receive the acceleration, the sensorcasing including a cylindrical fixed case member and a cover assemblyprovided on the fixed case member, the fixed case member having acircular bottom portion having a first circular inner surface, acylindrical side portion integrally formed with the bottom portion, anda supporting portion projecting from the bottom portion, the coverassembly having a circular cover portion having a second circular innersurface, and a cylindrical side portion integrally formed with the coverportion, the side portion of the sensor casing partly having a thirdcylindrical inner surface connected at one end with the first innersurface of the bottom portion, the side portion of the cover assemblypartly having the third cylindrical inner surface connected at the otherone end with the second inner surface of the cover portion, the firstinner surface of the bottom portion of the fixed case member, the secondinner surface of the cover portion of the cover assembly, the thirdinner surface of the side portion of the fixed case member, and thethird inner surface of the side portion of the cover assemblycollectively defining a cylindrical closed space; an oscillation plateaccommodated in the closed space of the sensor casing and having acentral portion supported by the supporting portion of the fixed casemember of the sensor casing, and a peripheral portion integrally formedwith the central portion and extending radially outwardly of the centralportion to be freely movable with respect to the supporting portion ofthe fixed case member of the sensor casing, the oscillation plate havinga first surface opposing to and spaced apart from the cover assembly anda second surface opposing to and spaced apart from the bottom portion ofthe fixed case member, the oscillation plate being partly oscillatablealong the center axis with respect to the fixed case member; and apiezoelectric element held in contact with the oscillation plate togenerate a voltage indicative of the acceleration when the accelerationis exerted on the sensor casing to have the oscillation plate partlyoscillated along the center axis with respect to the sensor casing withthe peripheral portion of the oscillation plate being deformed, thepiezoelectric element having first and second electrodes mountedthereon, the first electrode disposed between the piezoelectric elementand the oscillation plate, the second electrode opposing the coverassembly, the first and second electrodes allowing the voltageindicative of the acceleration to output therethrough, the oscillationplate having a thickness t₁ (mm), and an effective oscillation radius R₁(mm) measured between the inner and outer ends of the peripheral portionof the oscillation plate, wherein the ratio of the effective oscillationradius R₁ (mm) to the thickness t₁ (mm) of the oscillation plate may bemaintained within a fluctuation range of 3.3 plus minus 1.5, i.e., givenby the equation as follows,

R ₁ /t 1=3.3±1.5

In the above acceleration sensor, the piezoelectric element has aradially outer end, the peripheral portion of the oscillation plate hasa radially inner end, and the length between the radially outer end ofthe piezoelectric element and the radially inner end of the oscillationplate being R₂ (mm), wherein R₂ (mm) may be equal to 0.5 plus minus0.25, multiplied by R₁ (mm), i.e., given by the equation as follows,

R ₂=(0.5±0.25)R ₁

In the above acceleration sensor, the supporting portion of the fixedcase member has a cylindrical section and a forward tapered sectionintegrally formed with the cylindrical section and in the form of atruncated cone shape, the forward tapered section having a top surfacesecurely held in contact with the second surface of the oscillationplate and having an outer end edge in axially alignment with the outerperipheral end of the peripheral portion of the oscillation plate, theouter end edge having a diameter φC₁ (mm) and the piezoelectric elementbeing in the form of an annular shape to have an inner diameter φB₂(mm), whereby the inner diameter φB₂ (mm) of the annular shape of thepiezoelectric element is approximately equal to or greater than thediameter φC₁ (mm) of the outer end edge of the top surface of thesupporting portion.

In the above acceleration sensor, the piezoelectric element has athickness t₂ whereby the thickness t₁ (mm) of the oscillation plate isapproximately equal to the thickness t₂ (mm) of the piezoelectricelement, or the ratio of the thickness t₁ (mm) of the oscillation plateto the thickness t₂ of the piezoelectric element may be within afluctuation range of 0.5 to 3.

In accordance with a fourth aspect of the present invention, there isprovided an acceleration sensor for detecting an acceleration caused byan object oscillated in an oscillation direction, comprising a sensorcasing having a center axis and to be positioned in coaxial alignmentwith the oscillation direction to receive the acceleration, the sensorcasing including a cylindrical fixed case member and a cover assemblyprovided on the fixed case member, the fixed case member having acircular bottom portion having a first circular inner surface, acylindrical side portion integrally formed with the bottom portion, anda supporting portion projecting from the bottom portion, the coverassembly having a circular cover portion having a second circular innersurface, and a cylindrical side portion integrally formed with the coverportion, the side portion of the sensor casing partly having a thirdcylindrical inner surface connected at one end with the first innersurface of the bottom portion, the side portion of the cover assemblypartly having the third cylindrical inner surface connected at the otherone end with the second inner surface of the cover portion, the firstinner surface of the bottom portion of the fixed case member, the secondinner surface of the cover portion of the cover assembly, the thirdinner surface of the side portion of the fixed case member, and thethird inner surface of the side portion of the cover assemblycollectively defining a cylindrical closed space; an oscillation plateaccommodated in the closed space of the sensor casing and having acentral portion supported by the supporting portion of the fixed casemember of the sensor casing, and a peripheral portion integrally formedwith the central portion and extending radially outwardly of the centralportion to be freely movable with respect to the supporting portion ofthe fixed case member of the sensor casing, the oscillation plate havinga first surface opposing to and spaced apart from the cover assembly anda second surface opposing to and spaced apart from the bottom portion ofthe fixed case member, the oscillation plate being partly oscillatablealong the center axis with respect to the fixed case member; a firstpiezoelectric element having flat surfaces and provided on the firstflat surface of the oscillation plate to generate a voltage indicativeof the acceleration when the acceleration is exerted on the sensorcasing to have the oscillation plate partly oscillated along the centeraxis with respect to the sensor casing with the peripheral portion ofthe oscillation plate being deformed; a second piezoelectric elementhaving flat surfaces and provided on the second flat surface of theoscillation plate to generate a voltage indicative of the accelerationwhen the acceleration is exerted on the sensor casing to have theoscillation plate partly oscillated along the center axis with respectto the sensor casing with the peripheral portion of the oscillationplate being deformed, the first and second piezoelectric elements eachhaving a pair of electrodes having the voltage indicative of theacceleration to output therethrough, and the oscillation plate and thefirst and second piezoelectric elements being integrally oscillatablewithin a range of effective oscillation frequencies, the electrodesallowing the voltage indicative of the acceleration to outputtherethrough, the oscillation plate having a thickness t₁ (mm), and aneffective oscillation radius R₁ (mm) measured between the inner andouter ends of the peripheral portion of the oscillation plate, whereinthe ratio of the effective oscillation radius R₁ (mm) to the thicknesst₁ (mm) of the oscillation plate is maintained within a fluctuationrange of 3.3 plus minus 1.5, i.e., given by the equation as follows,

R ₁ /t 1=3.3±1.5.

In the above acceleration sensor, the piezoelectric element has aradially outer end, the peripheral portion of the oscillation plate hasa radially inner end, the length between the radially outer end of thepiezoelectric element and the radially inner end of the oscillationplate being R₂ (mm), wherein R₂ (mm) may be equal to 0.5 plus minus0.25, multiplied by R₁ (mm), i.e., given by the equation as follows,

R ₂=(0.5±0.25)R ₁

In the above acceleration sensor, the supporting portion of the fixedcase member has a cylindrical section and a forward tapered sectionintegrally formed with the cylindrical section and in the form of atruncated cone shape, the forward tapered section having a top surfacesecurely held in contact with the second surface of the oscillationplate and having an outer end edge in axially alignment with the outerperipheral end of the peripheral portion of the oscillation plate, theouter end edge having a diameter φC₁ (mm) and the piezoelectric elementbeing in the form of an annular shape to have an inner diameter φB₂(mm), whereby the inner diameter φB₂ (mm) of the annular shape of thepiezoelectric element may be approximately equal to or greater than thediameter φC₁ (mm) of the outer end edge of the top surface of thesupporting portion.

In the above acceleration sensor, the piezoelectric element has athickness t₂, whereby the thickness t₁ (mm) of the oscillation plate maybe approximately equal to the thickness t₂ (mm) of the piezoelectricelement, or the ratio of the thickness t₁ (mm) of the oscillation plateto the thickness t₂ of the piezoelectric element may be within afluctuation range of 0.5 to 3.

In accordance with a fifth aspect of the present invention, there isprovided an acceleration sensor for detecting an acceleration caused byan object oscillated in an oscillation direction, comprising a sensorcasing having a center axis and to be positioned in coaxial alignmentwith the oscillation direction to receive the acceleration, the sensorcasing including a cylindrical fixed case member having a supportingportion axially extending, and a cover assembly provided on the fixedcase member to cover the fixed case member to define a closed space; anoscillation plate accommodated in the closed space of the sensor casingand having a central portion supported by the supporting portion of thefixed case member, and a peripheral portion integrally formed with thecentral portion and extending radially outwardly of the central portionto be freely movable with respect to the supporting portion of the fixedcase member, the oscillation plate being partly oscillatable along thecenter axis with respect to the fixed case member, a piezoelectricelement held in contact with the oscillation plate to generate a voltageindicative of the acceleration when the acceleration is exerted on thesensor casing to have the oscillation plate partly oscillated along thecenter axis with respect to the sensor casing with the peripheralportion of the oscillation plate being deformed, the piezoelectricelement having a pair of electrodes having the voltage indicative of theacceleration to output therethrough, and the oscillation plate and thepiezoelectric element being integrally oscillatable within a range ofeffective oscillation frequencies; and at least one terminal pinextending through the cover assembly and terminating at the exterior ofthe cover assembly, the terminal pin electrically connected with one ofthe electrodes; whereby the oscillation plate and the piezoelectricelement may be integrally oscillatable in two different modes consistingof: a first oscillation mode where the oscillation plate is irregularlydeformed to have the peripheral portion oscillated with a single vectorin the oscillation direction of the oscillation plate when theoscillation plate is oscillated with respect to the fixed case member ata resonance frequency f₀; and a second oscillation mode where theoscillation plate is irregularly deformed to have two different halfparts of the peripheral portion oscillated with their respectivedifferent vectors opposite to each other in the oscillation direction ofthe oscillation plate when the oscillation plate is oscillated withrespect to the fixed case member at a noise frequency f₀ 1, and theresonance frequency f₀ and the noise frequency f₀ 1 may be out of therange of effective oscillation frequencies.

In the above acceleration sensor, the supporting portion of the fixedcase member has a cylindrical section and a forward tapered sectionintegrally formed with the cylindrical section and in the form of atruncated cone shape, the forward tapered section having a top surfacesecurely held in contact with the second surface of the oscillationplate and having an outer end edge in axially alignment with the outerperipheral end of the peripheral portion of the oscillation plate, theouter end edge having a diameter φC₁ (mm), and the oscillation platehaving an effective oscillation radius R₁ (mm) measured between theinner and outer ends of the peripheral portion of the oscillation plate;whereby the oscillation plate and the piezoelectric element may beintegrally oscillatable in the first and second oscillation modes withφC₁ (mm)/R₁ (mm) and f₀ 1/f₀ given in the following equation:

φC ₁ (mm)/R ₁ (mm)≧0.92

and

f ₀ 1 /f ₀≧0.52.

In the above acceleration sensor, the fixed case member and theoscillation plate are each made of an electroconductive material toensure that the remaining one of the electrodes is electricallyconnected with the oscillation plate and the fixed case member.

In accordance with a sixth aspect of the present invention, there isprovided an acceleration sensor for detecting an acceleration caused byan object oscillated in an oscillation direction, comprising a sensorcasing having a center axis and to be positioned in coaxial alignmentwith the oscillation direction to receive the acceleration, the sensorcasing including a cylindrical fixed case member having a supportingportion axially extending, and a cover assembly provided on the fixedcase member to cover the fixed case member to define a closed space; anoscillation plate accommodated in the closed space of the sensor casingand having a central portion supported by the supporting portion of thefixed case member, and a peripheral portion integrally formed with thecentral portion and extending radially outwardly of the central portionto be freely movable with respect to the supporting portion of the fixedcase member, the oscillation plate being partly oscillatable along thecenter axis with respect to the fixed case member, the oscillation platehaving a first flat surface opposing and spaced apart along the centeraxis with respect to the fixed case member, and a second flat surfaceopposing and spaced apart along the center axis with respect to thecover assembly of the sensor casing; a first piezoelectric elementhaving a first surface and a second surface, the first surface of thefirst piezoelectric element held in contact with the first flat surfaceof the oscillation plate to generate a voltage indicative of theacceleration when the acceleration is exerted on the sensor casing tohave the oscillation plate partly oscillated along the center axis withrespect to the sensor casing with the peripheral portion of theoscillation plate being deformed; a second piezoelectric element havinga first surface and a second surface, the first surface of the secondpiezoelectric element held in contact with the second flat surface ofthe oscillation plate to generate a voltage indicative of theacceleration when the acceleration is exerted on the sensor casing tohave the oscillation plate partly oscillated along the center axis withrespect to the sensor casing with the peripheral portion of theoscillation plate being deformed, the first and second piezoelectricelements each having a plurality of electrodes having the voltageindicative of the acceleration to output therethrough, the electrodesincluding a first electrode provided on the second surface of the firstpiezoelectric element, and a second electrode provided on the secondsurface of the second piezoelectric element, and the oscillation plateand the first and second piezoelectric elements being integrallyoscillatable within a range of effective oscillation frequencies; and atleast one terminal pin extending through the cover assembly andterminating at the exterior of the cover assembly, the terminal pinelectrically connected with the first and second electrodes; whereby theoscillation plate and the first and second piezoelectric elements may beintegrally oscillatable in two different modes consisting of: a firstoscillation mode where the oscillation plate is irregularly deformed tohave the peripheral portion oscillated with a single vector in theoscillation direction of the oscillation plate when the oscillationplate is oscillated with respect to the fixed case member at a resonancefrequency f₀; and a second oscillation mode where the oscillation plateis irregularly deformed to have two different half parts of theperipheral portion oscillated with their respective different vectorsopposite to each other in the oscillation direction of the oscillationplate when the oscillation plate is oscillated with respect to the fixedcase member at a noise frequency f₀ 1, and the resonance frequency f₀and the noise frequency f₀ 1 are out of the range of effectiveoscillation frequencies.

In the above acceleration sensor, the supporting portion of the fixedcase member has a cylindrical section and a forward tapered sectionintegrally formed with the cylindrical section and in the form of atruncated cone shape, the forward tapered section having a top surfacesecurely held in contact with the second surface of the oscillationplate and having an outer end edge in axially alignment with the outerperipheral end of the peripheral portion of the oscillation plate, theouter end edge having a diameter φC₁ (mm), and the oscillation platehaving an effective oscillation radius R₁ (mm) measured between theinner and outer ends of the peripheral portion of the oscillation plate;whereby the oscillation plate and the first and second piezoelectricelement may be integrally oscillatable in the first and secondoscillation modes with φC₁ (mm)/R₁ (mm) and f₀ 1/f₀ given in thefollowing equation:

φC ₁ (mm)/R ₁ (mm)≧0.92

and

f ₀ 1 /f ₀≧0.52.

In the above acceleration sensor, the first piezoelectric element has athird electrode provided on the first surface of the first piezoelectricelement, and second piezoelectric element has a fourth electrodeprovided on the first surface of the second piezoelectric element, andthe fixed case member and the oscillation plate are each made of anelectroconductive material and to ensure that the third electrode offirst piezoelectric element and the fourth electrode of the secondpiezoelectric element are electrically connected with the oscillationplate and the fixed case member.

In the above acceleration sensor, the cover assembly comprises a metalbase member made of an electroconductive material and a cover membermade of an insulating material, the metal base member having aperipheral end portion secured to part of the fixed case member with theclosed space defined by the metal base member and the fixed case member,the cover member being mounted on the metal base member, and theterminal pin extending through the metal base member and the covermember and electrically connected with the second electrode of thepiezoelectric element.

In the above acceleration sensor, the metal base member is formed with acentral hole, and the terminal pin having a contacting rod portionextending through the central hole of the metal base member to projectinto the closed space in electrical connection with the second electrodeof the piezoelectric element.

In the above acceleration sensor, the cylindrical side portion issmaller in diameter than the metal base member to form an annular gapbetween the inner surface of the small diameter portion of the fixedcase member and the outer peripheral end surface of the cover member,and which further comprises a resilient ring disposed in the annular gapand rested on the metal base member to have the closed spacehermetically sealed.

In the above acceleration sensor, the resilient ring is made of anO-ring.

In the above acceleration sensor, the metal base member having aperipheral end portion welded to part of the fixed case member.

In the above acceleration sensor, the fixed case member has a largediameter portion, a small diameter portion, and an annular ledge portionhaving the large and small diameter portions integrally formed with eachother to have the peripheral end portion of the metal case member firmlymounted thereon and welded thereto, the small diameter portion having anopen peripheral edge inwardly bent to be firmly engaged with the secondsurface of the cover member.

In the above acceleration sensor, the metal base member has a peripheralend portion secured to part of the fixed case member with the closedspace defined by the metal base member and the fixed case member. Thecover member is mounted on the metal base member, and the terminal pinextends through the metal base member and the cover member andelectrically connected with the one of the electrodes of thepiezoelectric element.

In the above acceleration sensor, the peripheral end portion of themetal base member is welded to the part of the fixed case member. Thefixed case member has a screw portion to be screwed to an exteriorobject which is to receive the acceleration. In the above accelerationsensor, the resonance frequency f₀ is 20 kHz or greater, and the rangeof effective oscillation frequencies is between 0 and 15 kHz.

BRIEF DESCRIPTION OF THE DRAWINGS

The feature and advantages of the present invention will become moreapparent from the following detailed description when considered inconnection with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view of a first embodiment of anacceleration sensor according to the present invention;

FIG. 2 is an enlarged cross-sectional view showing the constitutionelements and parts forming part of the acceleration sensor used todescribe how to assemble the acceleration sensor shown in FIG. 1;

FIG. 3 is a cross-sectional view similar to FIG. 1, but showing a secondembodiment of the acceleration sensor,

FIG. 4 is a cross-sectional view similar to FIG. 1, but showing any oneof third to seventh embodiments of the acceleration sensor;

FIG. 5 is an enlarged cross-sectional view showing the dimensions of anoscillation plate, a piezoelectric element, and a supporting portionforming parts of the acceleration sensor shown in FIG. 4;

FIG. 6 is a table showing constants of E (Young's modulus), ρ (density),and σ (Poisson's ratio) of the oscillation plate and the piezoelectricelement forming parts of the acceleration sensor;

FIG. 7 is an enlarged cross-sectional view similar to FIG. 5 but showingthe dimensions of the oscillation plate, the piezoelectric element, andthe supporting portion forming parts of the acceleration sensor used foractual measurements;

FIGS. 8A and 8B are graphs showing the relationship between thethickness t₁ and the sensitivity V₀/resonance frequency f₀;

FIG. 9 is an enlarged cross-sectional view similar to FIG. 5 but showingthe dimensions of the oscillation plate, the piezoelectric element, andthe supporting portion forming parts of the acceleration sensor used foractual measurements;

FIGS. 10A and 10B are graphs showing the relationship between theproportion of R₂/R₁ and the resonance frequency f₀;

FIG. 11 is an enlarged cross-sectional view similar to FIG. 5 butshowing the dimensions of the oscillation plate, the piezoelectricelement, and the supporting portion forming parts of the accelerationsensor used for actual measurements;

FIG. 12 is a graph showing the relationship between the proportion ofR₂/R₁ and the sensitivity V₀/the resonance frequency f₀;

FIG. 13 is an enlarged cross-sectional view similar to FIG. 5 butshowing the dimensions of the oscillation plate, the piezoelectricelement, and the supporting portion forming parts of the accelerationsensor used for actual measurements;

FIG. 14 is a table showing the relationship between the sensitivityV₀/the resonance frequency f₀, and the inner diameter φB₂ (mm) of thepiezoelectric element with respect to the diameter φC₁ (mm) of thesupporting portion;

FIG. 15 is a table showing the relationship between the sensitivityV₀/the resonance frequency f₀, and the proportion of the thickness t₁ ofthe oscillation plate with respect to the thickness t₂ of thepiezoelectric element;

FIG. 16 is an enlarged cross-sectional view similar to FIG. 5 butshowing the dimensions of the oscillation plate, the piezoelectricelement, and the supporting portion forming parts of the seventhembodiment of the acceleration sensor;

FIG. 17 is a table showing the result of experiments performed to provewhether spurious is eliminated or not;

FIG. 18 is a graph showing the result of experiments performed to provewhether spurious is eliminated or not;

FIG. 19 is a graph showing the result of experiments performed to provewhether spurious is eliminated or not;

FIG. 20 is a graph showing the result of experiments performed to provewhether spurious is eliminated or not;

FIG. 21 is a graph showing the result of experiments performed to provewhether spurious is eliminated or not;

FIG. 22 is a cross-sectional view showing a modification of any one ofthe third to seventh embodiments of the acceleration sensor according tothe present invention;

FIG. 23 is a cross-sectional view showing any one of eighth to twelfthembodiments of the acceleration sensor according to the presentinvention;

FIG. 24 is an enlarged cross-sectional view showing the constitutionelements and parts forming part of the acceleration sensor shown in FIG.23;

FIG. 25 is a cross-sectional view of a first conventional accelerationsensor;

FIG. 26 is an enlarged cross-sectional view showing the constitutionelements and parts forming part of the acceleration sensor shown in FIG.25;

FIG. 27 is a cross-sectional view of a second conventional accelerationsensor;

FIG. 28 is a graph showing the relationship between frequency f andoutput voltage V;

FIG. 29 is a cross-sectional view of a third conventional accelerationsensor;

FIG. 30 is an enlarged cross-sectional view showing the constitutionelements and parts forming part of the acceleration sensor shown in FIG.29; and

FIGS. 31A, 31B, and 31C are diagrams showing oscillation platesoscillating in the ½ oscillation mode, in the ½ oscillation mode, and inthe ¼ oscillation mode, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the acceleration sensor according to thepresent invention will now be described in detail in accordance with theaccompanying drawings.

Referring now to FIGS. 1 and 2 of the drawings, there is shown a firstpreferred embodiment of the acceleration sensor 200 assumed to beinstalled on an engine of an automotive vehicle. The acceleration sensor200 is shown in FIGS. 1 and 2 as comprising a sensor casing 250 having acenter axis and to be positioned in coaxial alignment with theoscillation direction to receive the acceleration. The sensor casing 250includes a cylindrical fixed case member 211 having a supporting portion211 a axially extending, and a cover assembly 240 provided on the fixedcase member 211 to cover the fixed case member 211 to define a closedspace 229. The acceleration sensor 200 further comprises an oscillationplate 212 accommodated in the closed space 229 of the sensor casing 250and having a central portion 212 a supported by the supporting portion211 a of the fixed case member 211, and a peripheral portion 212 bintegrally formed with the central portion 212 a and extending radiallyoutwardly of the central portion 212 a to be freely movable with respectto the supporting portion 211 a of the fixed case member 211. Theoscillation plate 212 is adapted to be partly oscillatable along thecenter axis with respect to the fixed case member 211.

In the second conventional acceleration sensor 110, the oscillationplate 102 is securely mounted on the metal base member 112 (see FIG.27), thereby causing the oscillation plate 102 to be oscillated togetherwith the metal base member 112 and the fixed case member 111, each ofwhich has a low resonance frequency. This leads to the fact that theresonance frequency f₀ of the oscillation plate 102 is lowered in theacceleration sensor 200 thus constructed, on the other hand, theoscillation plate 212 is not securely mounted on the cover assembly 240as shown in FIG. 1, thereby making it possible for the resonancefrequency f₀ of the oscillation plate 212 to prevent from being loweredbut to remain beyond a predetermined range of effective frequenciesactually used for detecting an acceleration.

The acceleration sensor 200 further comprises a piezoelectric element213 held in contact with the oscillation plate 212 to generate a voltageindicative of the acceleration when the acceleration is exerted on thesensor casing 250 to have the oscillation plate 212 partly oscillatedalong the center axis with respect to the sensor casing 250 with theperipheral portion 212 b of the oscillation plate 212 being deformed.The piezoelectric element 213 has first and second surfaces respectivelyhaving first and second electrodes 214 a, 214 b mounted thereon to havea voltage indicative of the acceleration to output therethrough. Thepiezoelectric element 213 is made of a piezoelectric material such asceramic.

The oscillation plate 212 is adapted to be oscillated when an exteriorobject such as an engine is oscillated. The oscillation of theoscillation plate 212 causes the piezoelectric element 213 to beoscillated and deformed to generate an electric charge Q. The outputvoltage V is outputted in accordance with a capacitance C of thepiezoelectric element 213 as given in the equation stated below. Theacceleration is then detected by measuring the output voltage V thusobtained.

V=Q/C

The first embodiment of the acceleration sensor 200 further comprises anoscillation plate 212 with the central portion 212 a securely supportedsimilar to the first and second conventional acceleration sensors 100and 110 and designed to have a resonance frequency f₀ of the oscillationplate 212 out of the predetermined range of effective oscillationfrequencies actually used for detecting an acceleration so that the flatportion V₀ of the output voltage range is used for detecting anacceleration (see FIG. 28).

The acceleration sensor 200 further comprises a terminal pin 217 bextending through the cover assembly 240 and terminating at the exteriorof the cover assembly 240, and a printed board 219 retained by the coverassembly 240 to have the second electrode 214 b of the piezoelectricelement 213 and the terminal pin 217 b connected with each other. Thisleads to the fact that the terminal pin 217 b serves as an outputterminal. The printed board 219 has a copper plated pattern 219 a on oneor both surfaces thereof.

In the first embodiment of the acceleration sensor 200, the fixed casemember 211 and the oscillation plate 212 are each made of anelectroconductive material to ensure that the first electrode 214 a iselectrically connected with the oscillation plate 212 and the fixed casemember 211. This means that the fixed case member 211 serves as aground. The cover assembly 240 comprises a metal base member 215 made ofan electroconductive material and a cover member 216 made of aninsulating material such as resin. The metal base member 215 has aperipheral end portion 215 b welded to part of the fixed case member 211with the closed space 229 defined by the metal base member 215 and thefixed case member 211. Thus, the metal base member 215, the cover member216, and the fixed case member 211 can be securely mounted by way of anautomatic assembly device. The oscillation plate 212 and thepiezoelectric element 213 are accommodated in the closed space 229 asshown in FIG. 2. The cover member 216 is mounted on the metal basemember 215 with the printed board 219 interposed between the metal basemember 215 and the cover member 216. The terminal pin 217 b extendsthrough the metal base member 215, the printed board 219 and the covermember 216. The printed board 219 is electrically connected with thesecond electrode 214 b of the piezoelectric element 213. The fact thatthe metal base member 215, the cover member 216, and the fixed casemember 211 can be securely mounted by way of an automatic assemblydevice, makes it easy for the acceleration sensor 200 to beautomatically assembled as shown in FIG. 2.

In the acceleration sensor 200, the metal base member 215 is formed witha central hole 215 a, and the terminal pin 217 b having a contacting rodportion 225 projecting from the copper plated pattern 219 a of theprinted board 219 and extending through the central hole 215 a of themetal base member 215 to project into the closed space 229 in electricalconnection with the second electrode 214 b of the piezoelectric element213.

The terminal pin 217 b electrically not in connection with the metalbase member 215 is electrically connected with the second electrode 214b of the piezoelectric element 213 through the contacting rod portion225. This enables the acceleration sensor 200 to detect an accelerationin a manner that the oscillation of the oscillation plate 212 causes thepiezoelectric element 213 to generate output voltage V, which isoutputted to the exterior connector, not shown, through the printedboard 219, the contacting rod portion 225, and the terminal pin 217 b.The contacting rod portion 225 may be replaced with other means, forinstance, a lead line electrically connecting the second electrode 214 bof the piezoelectric element 213 while the contacting rod portion 225 ofthe acceleration sensor 200 is appropriate for the automatic assembly ofthe acceleration sensor 200.

The printed board 219 has a signal processing circuit. The printed board219 may be replaced with a circuit soldered with the copper platedpattern 219 a, having an impedance converter circuit, an amplifiercircuit, and a resistance r for detecting a disconnection, connected inparallel with the piezoelectric element 213.

In the acceleration sensor 200, the cover member 216 has a first surfacefirmly held in contact with the metal base member 215 and a secondsurface open to the atmosphere. The fixed case member 211 has a largediameter portion 211 b, a small diameter portion 211 c, and an annularledge portion 211 d having the large and small diameter portions 211 band 211 c integrally formed with each other to have a peripheral endportion 215 b of the metal base member 215 mounted thereon and weldedthereto. The small diameter portion 211 c has an open peripheral edge211 n inwardly bent to be firmly engaged with the second surface of thecover member 216. This means that the metal base member 215 and thecover member 216 can be securely mounted by way of an automatic assemblydevice, thereby making it easy for the acceleration sensor 200 to beautomatically assembled.

In the acceleration sensor 200, the cover member 216 is smaller indiameter than the metal base member 215 to form an annular gap 201between the inner surface of the small diameter portion 211 c of thefixed case member 211 and the outer peripheral end surface of the covermember 216. The acceleration sensor 200 comprises a resilient ring 218disposed in the annular gap 201 and rested on the metal base member 215to have the closed space 229 hermetically sealed. Furthermore, theresilient ring 218 may be made of an O-ring. Here, the peripheral endportion 215 b of the metal base member 215 may not be welded to theannular ledge portion 211 d of the fixed case member 211. The the metalbase member 215 and the cover member 216 can be securely mounted by wayof an automatic assembly device, thereby making it easy for theacceleration sensor 200 to be automatically assembled.

In the acceleration sensor 200, the cover member 216 is formed with acircular recess 216 b and an annular groove 216 a open to the circularrecess 216 b. The printed board 219 is received in the circular recess216 b. The metal base member 215 is formed with an annular projection215 c extending through the printed board 219 and snugly received in theannular groove 216 a of the cover member 216 to have the cover member216 positioned with respect to the metal base member 215 with theprinted board 219 positioned interposed between the cover member 216 andthe metal base member 215. This leads to the fact that the metal basemember 215, cover member 216, and the printed board 219 are securelymounted and integrated with fixed case member 211 having the oscillationplate 12 and the piezoelectric element 13 received therein, therebyenabling to handle all of the aforesaid parts as one piece.

In the acceleration sensor 200, the fixed case member 211 has a screwportion 211 e to be screwed to an exterior object which is to receivethe acceleration to ensure that the oscillation plate 212 is oscillatedwith respect to the fixed case member 211 when the exterior object isoscillated.

The following description will be directed to how to design thepreviously described acceleration sensor 200 according to the presentinvention and how to determine the dimensions of the constitutionelements and parts forming part of the acceleration sensor 200.

The dimensions of the acceleration sensor 200 will be describedhereinlater. The resonance frequency f₀ of an oscillation bodyconsisting of the oscillation plate 212 and the piezoelectric element213 is given in the equation stated below. As seen from the equation,with the radius of the oscillation body unchanged, the thickness t isrequired to be increased in order to raise the resonance frequency f₀ ofthe oscillation body.

f ₀≈α′·(t/R ²)

where α′ stands for a constant.

The thickness t, however, has an upper limit. It is verified throughrepeated experiments that the resonance frequency f₀ has the maximumpoint. Experiment results indicate that the following three conditionsare required to be satisfied in order to increase both the resonancefrequency f₀ and the sensitivity V₀.

1. R₁/t1=radius of oscillation plate 212/thickness of oscillation plate212≈3.3

2. R₂/R₁=radius of piezoelectric element 213/radius of oscillation plate212≈0.5

3. t1/t2=thickness of oscillation plate 212/thickness of piezoelectricelement 213≈1

The sensitivity V₀ is given in the equation stated below. As seen fromthe equation, the sensitivity V₀ is enhanced as the radius R of theoscillation body is increased in contrary to the relationship betweenthe resonance frequency f₀ and the radius R of the oscillation bodyrepresented in the previous equation.

V ₀ ≈α′·R ²

As will be understood from the conditions 1, 2, and 3, the size andweight of the oscillation body is required to be extremely enlarged inorder to enhance both the resonance frequency f₀ and the sensitivity V₀.

The aforesaid first conventional acceleration sensor 100 comprises acover member 106 made of a resin material. This means that the sideportion of the cover member 106 of the acceleration sensor 100 isrequired to be thick, thereby making it difficult for the radius R₁ ofthe oscillation plate 12 to be increased as shown in FIG. 200. While onthe other hand, the first embodiment of the acceleration sensor 200comprises a fixed case member 211 made of a metal material, and anoscillation plate 212 having a peripheral end portion 212 b axiallyaligned with and spaced apart from the side portion of the fixed casemember 211. The thickness of the fixed case member 211 can be reducedwhile maintaining a required strength because of the fact that the fixedcase member 211 is made of a metal material. This leads to the fact thatthe first embodiment of the acceleration sensor 200 makes it possible tohave the maximum radius R₁ of the oscillation plate 212 in a manner thatthe thickness of the side portion of the fixed case member 211 isreduced with the total size of the acceleration sensor 200 not enlarged.

In the aforesaid second conventional acceleration sensor 110, the metalbase member 112 is oscillated together with the oscillation plate 102 asshown in FIG. 27, thereby decreasing the resonance frequency f₀. Thismakes it difficult for the acceleration sensor 110 with the oscillationplate 102 supported by the metal base member 112 to obtain the flatportion of the frequency characteristics within a predetermined range ofeffective frequencies, which does not include the resonance frequencyf₀. While on the other hand, the first embodiment of the accelerationsensor 200 comprises an oscillation plate 212 supported on thesupporting portion 211 a of the fixed case member 211. The accelerationsensor 200 thus constructed can prevent the oscillation plate 212 fromoscillating together with the cover assembly 240, which serves to coverthe fixed case member 211. This leads to the fact that the oscillationplate 212 of the acceleration sensor 200 can be oscillated with theresonance frequency f₀ not decreased by cover assembly 240, therebymaking it possible to have the resonance frequency f₀ out of thepredetermined range of effective oscillation frequencies actually usedfor detecting an acceleration. This leads further to the fact that theacceleration sensor 200 can detect an acceleration with the outputvoltage within the output voltage range of the flat portion V₀ as shownin FIG. 28.

Referring to FIG. 3 of the drawings, there is shown a second preferredembodiment of the acceleration sensor 210 assumed to be installed on anengine of an automotive vehicle. The first embodiment of theacceleration sensor 200 is one-terminal type. The present invention canprovide two-terminal type acceleration sensor. The second embodiment ofthe acceleration sensor 210 is two-terminal type. The sameconstitutional elements and parts are simply represented by the samereference numerals as those of the first embodiment of the accelerationsensor 200, and will be thus omitted from description for avoidingtedious repetition.

The acceleration sensor 210 further comprises first and second terminalpins 217 a and 217 b extending through the cover assembly 240 andterminating at the exterior of the cover assembly 240. The printed board219 retained by the cover assembly 240 is adapted to have electrodes 214a and 214 b of the piezoelectric element 213 and the terminal pins 217 aand 217 b connected with each other. The first terminal pin 217 aextends through the metal base member 215, the printed board 219 and thecover member 216 to be electrically connected with the first electrode214 a of the piezoelectric element 213 by way of the fixed case member211, the metal base member 215 and the printed board 219. This meansthat the first terminal pin 217 a serves as a ground. The secondterminal pin 217 b extends through the metal base member 215, theprinted board 219 and the cover member 216 to be electrically connecteddirectly with the second electrode 214 b of the piezoelectric element213. This means that the second terminal pin 217 b serves as an outputterminal.

In the acceleration sensor 210, the metal base member 215 is formed witha central hole 215 a, and the second terminal pin 217 b has thecontacting rod portion 225 projecting from the copper plated pattern 219a of the printed board 219 and extends through the central hole 215 a ofthe metal base member 215 to project into the closed space 229 inelectrical connection with the second electrode 214 b of thepiezoelectric element 213.

The copper plated pattern 219 a′ of the printed board 219 is held incontact with the metal base member 215, thereby enabling to electricallyconnect the first terminal pin 217 a with the metal base member 215soldered at 219 a′, the fixed case member 211, the oscillation plate 212and the first electrode 214 a of the piezoelectric element 213, while onthe other hand the second terminal pin 217 b electrically not inconnection with the metal base member 215 is electrically connected withthe second electrode 214 b of the piezoelectric element 13 through thecontacting rod portion 225. This enables the acceleration sensor 210 todetect an acceleration in a manner that the oscillation of theoscillation plate 212 causes the piezoelectric element 213 to generateoutput voltage V, which is outputted to the exterior connector throughthe printed board 219, the contacting rod portion 225, and the secondterminal pin 217 b.

In the second embodiment of the acceleration sensor 210, the oscillationbody consisting of the oscillation plate 212 and the piezoelectricelement 213 are supported by the supporting portion 211 a formed on thecentral part of the bottom portion of the fixed case member 211. Thefixed case member 211 has an annular ledge portion 211 d and an openperipheral end portion 211 c. The metal base member 215 has an openperipheral portion 215 b welded on the annular ledge portion 211 d ofthe fixed case member 211. The open peripheral end portion 211 c of thefixed case member 211 is inwardly bent to be firmly engaged with thecover member 216 to define a closed space 229 having the oscillationbody received therein. The first and second terminal pins 217 a and 217b extending through cover member 216 are electrically connected with theprinted board 219 in a space 215 a formed by the metal base member 215to serve as ground and output terminals, respectively. The printed board219 is designed to electrically connect between the contacting rodportion 225 and the detection electrode 214 b of the piezoelectricelement 213. This leads to the fact that the detection electrode 214 bof the piezoelectric element 213 is not required to be soldered to awire.

The second conventional acceleration sensor 110 comprises an oscillationplate 102 and a piezoelectric element 103 supported by the supportingportion 112 a integrally formed with the metal base member 112, whichserves to cover the fixed case 111 as shown in FIG. 27. As describedhereinbefore, the performance of the second conventional accelerationsensor 110 thus constructed may be deteriorated due to temperature.While on the other hand, the acceleration sensor 200 comprises anoscillation plate 212 and a piezoelectric element 213 not supported bythe supporting portion integrally formed with the metal base member 215,which serves to cover the fixed case member 211, thereby preventing theacceleration sensor 200 from deteriorating the performance due to thetemperature characteristics. This leads to the fact that the first andsecond embodiments of the acceleration sensors 200 are of highperformance and appropriate for automatic production at a low cost.

The third conventional acceleration sensor 120, which comprises theweight 122 for pressuring the piezoelectric element 123 as shown in FIG.29, has many constitutional elements and parts, and is required to belarge in size. Furthermore, the assembly of the acceleration sensor 120requires works with a high degree of precision. While on the other hand,the first or second embodiments of the acceleration sensor 200 does notcomprise a weight, has constitutional parts fewer than the accelerationsensor 120, and is therefore not required to be large in size.Furthermore, the assembly of the acceleration sensor 200 does notrequire works with a high degree of precision, thereby facilitating theautomatic assembly of the acceleration sensor 200.

The acceleration sensor 200 comprises an oscillation plate 212 and apiezoelectric element 213 accommodated in the closed space 229hermetically sealed.

The construction of the acceleration sensor 210 shown in FIG. 3 is thesame as that of the acceleration sensor 200 shown in FIG. 1. It is thusto be noted that the operation and performance of the accelerationsensor 210 is also the same as those of the acceleration sensor 200.

The metal base member 215, for instance, may be provided with a hole toensure that the acceleration sensor functions as an audio converter forultrasound and the like.

As will be seen from the foregoing description, the acceleration sensoraccording to the present invention is of high performance and can beproduced at a low cost. The reason is due to the fact that the fixedcase member 211 and the cover assembly 240 collectively define a closedspace 229 accommodating the oscillation plate 212 and the piezoelectricelement 213 received therein, the oscillation plate 212 and thepiezoelectric element 213 are oscillatably supported by the supportingportion 211 a formed on the central bottom portion of the fixed casemember 211, and the second electrode 214 b of the piezoelectric element213 is electrically connected with the exterior connector through theprinted board 219 and the terminal pin 217 b, thereby reducing thenumber of the constitutional elements and parts and facilitating theautomatic assembly.

Referring also to FIGS. 4 and 5 of the drawings, there is shown a thirdpreferred embodiment of the acceleration sensor 300 according to thepresent invention. The acceleration sensor 300 is assumed to beinstalled on an engine of an automotive vehicle.

The acceleration sensor 300 is shown in FIG. 4 as comprising a sensorcasing 350 having a center axis and to be positioned in coaxialalignment with the oscillation direction to receive the acceleration.The sensor casing 350 includes a cylindrical fixed case member 311 and acover assembly 340 provided on the fixed case member 311. The fixed casemember 311 has a circular bottom portion 311 g having a first circularinner surface, a cylindrical side portion 311 h integrally formed withthe bottom portion 311 g, and a supporting portion 311 a projecting fromthe bottom portion 311 g. The cover assembly 340 has a circular coverportion 346 having a second circular inner surface, and a cylindricalside portion 345 integrally formed with the cover portion 346. The sideportion 311 h of the fixed case member 311 partly has a thirdcylindrical inner surface connected at one end with the first innersurface of the bottom portion 311 g. The side portion 345 of the coverassembly 340 partly has a third cylindrical inner surface connected atthe other one end with the second inner surface of the cover portion346. The first inner surface of the bottom portion 311 g of the fixedcase member 311, the second inner surface of the cover portion 346 ofthe cover assembly 340, the third inner surface of the side portion 311h of the fixed case member 311, and the third inner surface of the sideportion 345 of the cover assembly 340 collectively define a cylindricalclosed space 360.

The acceleration sensor 300 further comprises an oscillation plate 312accommodated in the closed space 360 of the sensor casing 350. Theoscillation plate 312 has a central portion 312 a supported by thesupporting portion 311 a of the fixed case member 311 of the sensorcasing 350, and a peripheral portion 312 b integrally formed with thecentral portion 312 a and extending radially outwardly of the centralportion 312 a to be freely movable with respect to the supportingportion 311 a of the fixed case member 311 of the sensor casing 350. Theoscillation plate 312 has a first surface opposing to and spaced apartfrom the cover assembly 340 and a second surface opposing to and spacedapart from the bottom portion 311 g of the fixed case member 311. Theoscillation plate 312 is adapted to be partly oscillatable along thecenter axis with respect to the fixed case member 311.

The acceleration sensor 300 further comprises a piezoelectric element313 held in contact with the oscillation plate 312 to generate a voltageindicative of the acceleration when the acceleration is exerted on thesensor casing 350 to have the oscillation plate 312 partly oscillatedalong the center axis with respect to the sensor casing 350 with theperipheral portion 312 b of the oscillation plate 312 being deformed.The piezoelectric element 313 has first and second electrodes 314 a, 314b mounted thereon. The first electrode 314 a is disposed between thepiezoelectric element 313 and the oscillation plate 312, and the secondelectrode 314 b is disposed opposing the cover assembly 340. The firstand second electrodes 314 a and 314 b are adapted to allow the voltageindicative of the acceleration to output therethrough. As shown in FIG.5, the oscillation plate 312 has a thickness t₁ (mm), and an effectiveoscillation radius R₁ (mm) measured between the inner end 312 d andouter end 312 c of the peripheral portion 312 b of the oscillation plate312. The ratio of the effective oscillation radius R₁ (mm) to thethickness t₁ (mm) of the oscillation plate 312 is maintained within afluctuation range of 3.3 plus minus 1.5, i.e., 3.3±1.5.

The acceleration sensor 300 according to the present invention has anoscillation plate 312 and a piezoelectric element 313 mounted on thesupporting portion 311 a of the fixed case member 311 with the centeraxes of the oscillation plate 312 and the piezoelectric element 313 heldin axial alignment with the center axis of the supporting portion 311 aof the fixed case member 311 so as to detect an acceleration. This leadsto the fact that a weight and a fastening screw can be omitted tosimplify the construction of the acceleration sensor, thereby making itpossible to automatically assemble the acceleration sensor, and reducethe cost of producing the acceleration sensor.

The following description will be directed how to design theacceleration sensor 300 according to the present invention and how todetermine the dimensions of the constitution elements and parts formingpart of the acceleration sensor 300.

The numeral setting that the ratio of the effective oscillation radiusR₁ (mm) to the thickness t₁ (mm) of the oscillation plate 312 ismaintained within a fluctuation range of 3.3 plus minus 1.5, i.e.,3.3±1.5 is intended to keep the resonance frequency f₀ of theoscillation plate 312 involving the piezoelectric element 313 above 20kHz. The reason for the numeral setting will be described in detailshereinlater.

When the acceleration sensor 300 receives an acceleration (G), theoscillation plate 312 is exerted with force (F) that induces oscillationof the oscillation plate 312 with its peripheral portion 312 b peaked.The oscillation of the oscillation plate 312 causes the piezoelectricelement 313 to be energized, thereby bringing about an electric signalindicative of a certain level of voltage. The cover assembly 340 has anoutput terminal 317 b mounted thereon. The voltage level thus generatedis then outputted from the detection electrodes 314 a and 314 b throughthe output terminal 317 b to ensure that the acceleration is detected bythe acceleration sensor 300.

The sensitivity V₀ of the acceleration sensor 300 is roughly inproportion to the square of the effective oscillation radius R of theoscillation plate 312 as described hereinbefore. This means that theoscillation plate 312 having a large effective oscillation radius R isadvantageous. The resonance frequency f₀ is inclined to rise in responseto the thickness t1 of the oscillation plate 312 up to a certain extentas described hereinbefore. However, if the proportion R₁/t1 of theeffective oscillation radius R₁ of the oscillation plate 312 withrespect to the thickness t1 of the oscillation plate 312 goes beyond acertain threshold level, the oscillation plate 312 does not function asan oscillation plate any more, thereby causing the sensitivity V₀ andthe resonance frequency f₀ to be lowered.

The proportion R₁/t1 of the effective oscillation radius R₁ of theoscillation plate 312 with respect to the thickness t1 of theoscillation plate 312 is therefore required to be maintained within acertain fluctuation range of 1.5 plus and minus from 3.3, i.e., given bythe equation as follows,

 R ₁ /t 1=3.3±1.5

The proportion R₁/t1 thus set ensures to have both the sensitivity V₀and resonance f₀ enhanced. The reason that the fluctuation range shouldbe specified in the range of 1.5 plus and minus from 3.3 will bedescribed in more detail.

The acceleration sensor is operated under two different contradictoryconditions consisting of a first condition that the sensitivity V₀enhances as the diameter of the oscillation plate 312 increases and asecond condition that the resonance frequency f₀ rises as the diameterof the oscillation plate 312 decreases. Accordingly, it is required tospecify an optimal value for the resonance frequency f₀ appropriate forthe acceleration sensor 300. The optimal resonance frequency f₀ will beestimated from measured pieces of data obtained through repeatedexperiments, hereinlater. The relationship between the resonancefrequency f₀ and the flat portion V₀ of the output voltage range is asfollows (the values may change to some extent depending on the outputlevel V₀).

(1) The flat portion of the output voltage range should lie at afrequency in a range of less than 2 octaves (a quarter) of the resonancefrequency f₀.

(2) The output voltage level at a frequency of 1 octave of the resonancefrequency f₀ should lie in a range of +3 dB of the flat portion of theoutput voltage level.

In view of the case of (1), the resonance frequency f₀ is required to be30 (kHz) in order to obtain the flat portion of the output voltage rangeat a frequency of, for instance, 15 (kHz), while the resonance frequencyf₀ is required to be 20 (kHz) in order to obtain the flat portion of theoutput voltage range at a frequency of 10 (kHz).

In general, the acceleration sensor of the non-resonance type has theeffective oscillation range of around 10 to 15 (kHz) or less actuallyused for detecting an acceleration. As a result, the output voltagelevel at a frequency in a range of 10 (kHz) or greater is required toform the flat portion. This leads to the fact that the resonancefrequency f₀ is desirable to be 20 (kHz) or greater. The proportion ofR₁/t₁ required to obtain optimal combination of f₀ and V₀ is determinedthrough repeated measurements, which will be described hereinlater.

FIG. 6 shows constants of E (Young's modulus), ρ (density), and σ(Poisson's ratio) of the oscillation plate 312 and the piezoelectricelement 313 obtained by the measurements.

As described hereinbefore, the resonance frequency f₀ of the oscillationplate in the form of a disc shape and securely mounted on the centralportion of the fixed case member is calculated by the aforesaid equation(1).

[Eq. 1]

f ₀=α(t/R ²)×{square root over (E/ρ(1−σ²))}  (1)

where α is 0.172, t and R stand for thickness and radius of theoscillation plate, respectively.

The constants of E, ρ, and σ may change to some extent depending on thematerial of the oscillation plate 312 and the piezoelectric element 313.The change of the constants of E, ρ, and σ, however, will notsignificantly affect the inner value of the square root in the equation(1), i.e., E/ρ(1−σ²) as confirmed by the calculation of the inner valueof the square root. As calculated from the equation (1), it is thereforeunderstood that the resonance frequency f₀ is affected by the dimensionsof constituting elements of the oscillation plate 312 rather than theaforesaid constants such as E, ρ, and σ.

The above factors f₀, t, R thus calculated may not completely satisfythe condition of the proportion of R₁/t₁, however, only to some extent.

FIG. 7 shows a sample of the supporting portion 311 a, the oscillationplate 312, and the piezoelectric element 313 used for the measurements.The measurements were carried out with the diameter φA₁ of theoscillation plate 312 is 22 (mm), the outer diameter φA₂ of thepiezoelectric element 313 is 13 (mm), the inner diameter φB₂ of thepiezoelectric element is 3.1 (mm), and the thickness t₂ of thepiezoelectric element 313 is 2 (mm).

FIG. 8A shows the relationship between the thickness t₁ and thesensitivity V₀/resonance frequency f₀ obtained by the measurementperformed under the condition that R₁=8.85 (φC₁=4.3 (mm)). FIG. 8B showsthe relationship between the thickness t₁ and the sensitivityV₀/resonance frequency f₀ obtained by the measurement performed underthe condition that R₁=8.15 (φC₁=5.7 (mm)).

From the measured pieces of data shown in FIGS. 8A and 8B, it isunderstood that R₁/t₁ should be around 3.3 in order to increase both ofV₀ and f₀. In view of the effective oscillation frequencies availablefor detecting an acceleration, R₁/t₁ is estimated to be in a range of3.3±1.5, even accepting the fact that the corresponding V₀ and f₀ may bereduced to a degree that can be tolerated.

From the measured pieces of data obtained through repeated experiments,it is also verified that maximum values of V₀ and f₀ will changedepending on the diameter φC₁ of the supporting portion 311 a, and thatf₀ will neither satisfy the equation (1) nor increase after thethickness t₁ exceeds a certain value. This means that f₀ has a certainupper limit.

As will be seen from the foregoing description, the third embodiment ofthe acceleration sensor 300 comprises a fixed case member 311, asupporting portion 311 a integrally formed with the fixed case member311 and provided on the central bottom portion of the fixed case member311, an oscillation plate 312 having a central portion 312 a firmlyconnected with the supporting portion 311 a, a piezoelectric element 313provided on a surface of the oscillation plate 312 having upper andlower surfaces on which the electrodes 314 a and 314 b are securelymounted, and a cover assembly 340 having an output terminal 317 bmounted thereon and electrically connected with the first electrode 314b to output an electric signal from the electrode 314 b. The thicknessand the effective oscillation radius of the oscillation plate 312 of theacceleration sensor 300 is t₁ and R_(i), respectively. The accelerationsensor 300 is designed in a manner that the proportion R₁/t₁ ismaintained in a fluctuation range of 1.5 plus and minus from 3.3,thereby making it possible to keep the resonance frequency f₀ of theoscillation plate 312 including the piezoelectric element 313 in a rangeof 20 kHz or greater. This leads to the fact that the sensitivity V₀ andthe resonance frequency f₀ of the acceleration sensor 300 is optimizedand the performance of the acceleration sensor 300 is enhanced.

The foregoing third embodiment of the acceleration sensor 300 can bereplaced by a fourth embodiment of the acceleration sensor 300 in orderto improve the performance of the acceleration sensor 300.

The third embodiment of the acceleration sensor 300 can be modified as afourth embodiment of the acceleration sensor 300 in a manner that thatthe dimensions of the oscillation plate 312, the piezoelectric element313 and the supporting portion 311 a of the fixed case member 311 aremodified. The fourth embodiment of the acceleration sensor 300 as amodification of the third embodiment of the acceleration sensor 300 willbe described hereinafter. The constitutional elements and parts of thefourth embodiment of the acceleration sensor 300 same as those of thethird embodiment of the acceleration sensor 300 are simply representedby the same reference numerals as those of the third embodiment of theacceleration sensor 300, and will be thus omitted from description foravoiding tedious repetition.

Referring to FIGS. 4 to 5 of the drawings, there is shown a fourthpreferred embodiment of the acceleration sensor 300 according to thepresent invention.

In the fourth embodiment of the acceleration sensor 300, thepiezoelectric element 313 has a radially outer end 313 c as shown inFIG. 5. The peripheral portion 312 b of the oscillation plate 312 has aradially inner end 312 d. Assuming that the length between the radiallyouter end 313 c of the piezoelectric element 313 and the radially innerend 312 d of the oscillation plate 312 is R₂ (mm). R₂ (mm) is equal to0.5 plus minus 0.25, multiplied by R₁ (mm), i.e., given in the equationas follows:

R ₂=(0.5±0.25)R ₁

The reason for the above equation will be described hereinlater indetail.

The calculated values of E/ρ(1−σ²) of the oscillation plate 312, whichis made of nickel steel, and the piezoelectric element 313 are asfollows.

Nickel steel . . . 5.28×10³

Piezoelectric element . . . 3.05×10³

As the results of the calculation from the equation (1), the inner valueof the square root in the equation (1), i.e., E/ρ(1−σ²) of thepiezoelectric element 313 is less than that of the oscillation plate312, which is made of nickel steel.

From the above calculation, it is derived that the resonance frequencyof the piezoelectric element 313 is approximately 0.578 times as high asthat of the oscillation plate 312, provided that the oscillation plate312 and the piezoelectric element 313 have the same dimension. Thismeans that the resonance frequency of the piezoelectric element 313tends to be lower than that of the oscillation plate 312. This leads tothe fact that the relatively low resonance frequency of thepiezoelectric element 313 will cause to decrease the resonance frequencyf₀ of the oscillation body consisting of the oscillation plate 312 andthe piezoelectric element 313.

The factor R₂ is calculated in accordance with the equation (1) underthe condition that the thickness t₁ of the oscillation plate 312 isequal to the thickness t₂ of the piezoelectric element 313 and theoscillation plate 312 and the piezoelectric element 313 have the sameresonance frequency f₀ as follows.

[Eq. 2]

R ₂={square root over (0.578)}R ₁  (2)

Accordingly, it is derived that R₂ of the piezoelectric element 313 isapproximately 0.76 times as large as R₁ of the oscillation plate 312. Inaddition, the resonance frequency of the piezoelectric element 313 isrequired to be twice as high as that of the oscillation plate 312 sothat the resonance frequency of the piezoelectric element 313 will notaffect that of the oscillation plate 312. R₂ of the piezoelectricelement 313 satisfying the above condition is calculated as below.

[Eq. 3]

R ₂={square root over (0.578/2)}R ₁  (3)

Accordingly, it is derived that R₂ of the piezoelectric element 313 isapproximately 0.538 times as large as R₁ of the oscillation plate 312.

On the other hand, the sensitivity V₀ is given in accordance with thefollowing equation as described hereinbefore.

V ₀ =Q/C

where Q stands for electrical charge, and C stands for capacitance.

The piezoelectric element 313 receives stress on the central area 313 dmore than on the peripheral area 313 f as shown in FIG. 5. Thepiezoelectric element 313 is therefore considered to generate electricalcharge on the central area 313 d more than on the peripheral area 313 f.The capacitance C increases on the peripheral area 313 f. For the abovereasons, it is inferred that the sensitivity V₀ is decreased on theperipheral area 313 f of the piezoelectric element 313 as shown in FIG.5. The outer diameter of the piezoelectric element 313 is accordinglydesirable to be shortened to enhance the sensitivity V₀.

As a result of the foregoing description, it is concluded that the outerdiameter of the piezoelectric element 313 should be smaller than that ofthe oscillation plate 312 in order to improve the performance.

FIGS. 10A and 10B shows the relationship between the proportion of R₂/R₁and the resonance frequency f₀ obtained by the measurement performedunder the condition that the outer diameter φA₁ of the oscillation plate312 is 22 (mm) and the thickness t₂ of the piezoelectric element 313 is2 (mm) as shown in FIG. 9. In FIG. 10A, R₁ is 8.85 (φC₁=4.3 (mm)), andin FIG. 10B, R₁ is 8.15 (φC₁=5.7 (mm)). As seen from FIG. 10, R₂<0.75 R₁is desirable in order to increase the resonance frequency f₀.

In order to determine the maximum value of the sensitivity V₀, thesensitivity V₀ is measured under the condition that the thickness t₁ ofthe oscillation plate 312 is 3 (mm), the thickness t₂ of thepiezoelectric element 313 is 2 (mm), and the diameter φC₁ of thesupporting portion 311 a is 4.3 (mm) as shown in FIG. 11. FIG. 12 showsthe relationship between the proportion of R₂/R₁ and the sensitivityV₀/the resonance frequency f₀ obtained by the measurement. As seen fromFIG. 12, R₂ is desirable to be 0.4 to 0.5 multiplied by R₁ in order toincrease the resonance frequency f₀.

From the foregoing description, it is concluded that R₁ and R₂ aredesirable to satisfy the equation as follows.

R ₂=(0.5±0.25)R ₁

where R₁ (mm) is the effective oscillation radius measured between theinner end 312 d and outer end 312 c of the peripheral portion 312 b ofthe oscillation plate 312, and R₂ (mm) is the length between theradially outer end 313 c of the piezoelectric element 313 and theradially inner end 312 d of the oscillation plate 312.

The fourth embodiment of the acceleration sensor 300 thus constructedcan enhance both the resonance frequency f₀ and the sensitivity V₀,thereby making it possible to improve the performance of theacceleration sensor 300.

The fourth embodiment of the acceleration sensor 300 has, however, adrawback that the adhesive area between the piezoelectric element 313and the oscillation plate 312 is curtailed, thereby causing the adhesivestrength between the piezoelectric element 313 and the oscillation plate312 to be unevenly distributed and weakened.

The foregoing third and fourth embodiments of the acceleration sensor300 can be replaced by a fifth embodiment of the acceleration sensor 300in order to improve the performance of the acceleration sensor 300.

The third and fourth embodiments of the acceleration sensor 300 can bemodified as a fifth embodiment of the acceleration sensor 300 in amanner that that the dimensions of the oscillation plate 312, thepiezoelectric element 313 and the supporting portion 311 a of the fixedcase member 311 are modified. The fifth embodiment of the accelerationsensor 300 as a modification of the third and fourth embodiments of theacceleration sensor 300 will be described hereinafter. Theconstitutional elements and parts of the fifth embodiment of theacceleration sensor 300 same as those of the third embodiment of theacceleration sensor 3100 are simply represented by the same referencenumerals as those of the third embodiment of the acceleration sensor300, and will be thus omitted from description for avoiding tediousrepetition.

Referring to FIGS. 4 and 5 of the drawings, there is shown a fifthpreferred embodiment of the acceleration sensor 300 according to thepresent invention. In the fifth embodiment of the acceleration sensor300, the supporting portion 311 a of the fixed case member 311 has acylindrical section 311 i and a forward tapered section 311 j integrallyformed with the cylindrical section 311 i and in the form of a truncatedcone shape as shown in FIG. 5. The forward tapered section 311 j has atop surface 311 k securely held in contact with the second surface ofthe oscillation plate 312 and has an outer end edge 311 l in axiallyalignment with the outer peripheral end 312 c of the peripheral portion312 b of the oscillation plate 312. The outer end edge 311 l has adiameter φC₁ (mm). The piezoelectric element 313 is in the form of anannular shape to have an inner diameter φB₂ (mm). The inner diameter φB₂(mm) of the annular shape of the piezoelectric element 313 isapproximately equal to or greater than the diameter φC₁ (mm) of theouter end edge 311 l of the top surface 311 k of the supporting portion311 a.

In the fifth embodiment of the acceleration sensor 300, thepiezoelectric element 313 in the form of a doughnut and disc shape ismounted on the oscillation plate 312 under the condition the innerdiameter φB₂ (mm) of the piezoelectric element 313 is optimized, inother words, the inner diameter φB₂ (mm) of the piezoelectric element313 is approximately equal to or greater than the diameter φC₁ (mm) ofthe supporting portion of the oscillation plate 312.

FIG. 14 shows the relationship between the sensitivity V₀/the resonancefrequency f₀ and the inner diameter φB₂ (mm) of the piezoelectricelement 313, with respect to the diameter φC₁ (mm) of the supportingportion 311 a obtained by the measurement performed under the conditionthat the outer diameter φA₁ (mm) of the oscillation plate 312 is 22.

As shown in FIG. 14, the inner diameter φB₂ (mm) of the piezoelectricelement 313 is desirable to be approximately equal to or greater thanthe diameter φC₁ (mm) of the supporting portion of the oscillation plate312 in order to improve the sensitivity V₀, even accepting the fact thatthe resonance frequency f₀ may be reduced to a degree that can betolerated. In addition, as the inner diameter of the piezoelectricelement 313, φB₂, increases, the electrical charge Q tends to increase(not shown). The inner diameter of the piezoelectric element 313, φB₂,accordingly, can increase only up to a certain limit since the adhesivearea is decreased and the sensitivity V₀ is deteriorated.

As will be seen from the foregoing description, the fifth embodiment ofthe acceleration sensor 300 has a piezoelectric element 313 in the formof an annular shape mounted on the oscillation plate 312 under thecondition that the inner diameter φB₂ (mm) of the piezoelectric element313 is approximately equal to or greater than the diameter φC₁ (mm) ofthe supporting portion of the oscillation plate 312, thereby making itpossible to enhance the sensitivity V₀, even accepting the fact that theelectrical charge Q is increased and the resonance frequency f₀ islowered to a degree that can be tolerated.

The foregoing third to fifth embodiments of the acceleration sensor 300can be replaced by a sixth embodiment of the acceleration sensor 300 inorder to improve the performance of the acceleration sensor 300.

The third to fifth embodiments of the acceleration sensor 300 can bemodified as a sixth embodiment of the acceleration sensor 300 in amanner that that the dimensions of the oscillation plate 312, thepiezoelectric element 313 and the supporting portion 311 a of the fixedcase member 311 are modified. The sixth embodiment of the accelerationsensor 300 as a modification of the third to fifth embodiments of theacceleration sensor 300 will be described hereinafter. Theconstitutional elements and parts of the sixth embodiment of theacceleration sensor 300 same as those of the third embodiment of theacceleration sensor 300 are simply represented by the same referencenumerals as those of the third embodiment of the acceleration sensor300, and will be thus omitted from description for avoiding tediousrepetition.

Referring to FIGS. 4 and 5 of the drawings, there is shown a sixthpreferred embodiment of the acceleration sensor 300 according to thepresent invention.

In the sixth embodiment of the acceleration sensor 300, thepiezoelectric element 313 has a thickness t₂. The thickness t₁ (mm) ofthe oscillation plate 312 is approximately equal to the thickness t₂(mm) of the piezoelectric element 313, or the ratio of the thickness t₁(mm) of the oscillation plate 312 to the thickness t₂ of thepiezoelectric element 313 is within a fluctuation range of 0.5 to 3 asshown 5.

In the sixth embodiment of the acceleration sensor 300 thus constructed,the thickness t₁ of the oscillation plate and the thickness t₂ of thepiezoelectric element are optimized.

FIG. 15 shows the relationship between the sensitivity V₀/the resonancefrequency f₀, and the proportion t₁/t₂ of the thickness t₁ of theoscillation plate 312 with respect to the thickness t₂ of thepiezoelectric element 313 obtained by measurements.

As shown in FIG. 15, the proportion t₁/t₂ is desirable to beapproximately equal to one (t₁=t₂) or in a range of 0.5 to 3 in order toobtain an optimized combination of f₀ and V₀.

As will be seen from the foregoing description, the sixth embodiment ofthe acceleration sensor 300 has a piezoelectric element 313 in the formof an annular shape mounted on the oscillation plate 312 under thecondition that the thickness t₁ (mm) of the oscillation plate 312 isapproximately equal to the thickness t₂ (mm) of the piezoelectricelement 313, or the ratio of the thickness t₁ (mm) of the oscillationplate 312 to the thickness t₂ of the piezoelectric element 313 is withina fluctuation range of 0.5 to 3, thereby making it possible to obtain anoptimized combination of the sensitivity V₀ and the resonance frequencyf₀.

The foregoing third to sixth embodiments of the acceleration sensor 300may be replaced by a seventh embodiment of the acceleration sensor 300in order to improve the performance of the acceleration sensor 300. Thethird to sixth embodiments of the acceleration sensor 300 can bemodified as a seventh embodiment of the acceleration sensor 300 in amanner that that the dimensions of the oscillation plate 312, thepiezoelectric element 313 and the supporting portion 311 a of the fixedcase member 311 are modified. The seventh embodiment of the accelerationsensor 300 as a modification of the third to sixth embodiments of theacceleration sensor 300 will be described hereinafter. Theconstitutional elements and parts of the seventh embodiment of theacceleration sensor 300 same as those of the third embodiment of theacceleration sensor 300 are simply represented by the same referencenumerals as those of the third embodiment of the acceleration sensor300, and will be thus omitted from description for avoiding tediousrepetition.

Referring to FIGS. 4 and 16 of the drawings, there is provided a seventhpreferred embodiment of the acceleration sensor 300 according to thepresent invention. The seventh embodiment of the acceleration sensor 300is shown in FIG. 4 as comprising a sensor casing 350 having a centeraxis and to be positioned in coaxial alignment with the oscillationdirection to receive the acceleration. The sensor casing 350 includes acylindrical fixed case member 311 having a supporting portion 311 aaxially extending, and a cover assembly 340 provided on the fixed casemember 311 to cover the fixed case member 311 to define a closed space360.

The acceleration sensor 300 further comprises an oscillation plate 312accommodated in the closed space 360 of the sensor casing 350 and has acentral portion 312 a supported by the supporting portion 311 a of thefixed case member 311, and a peripheral portion 312 b integrally formedwith the central portion 312 a and extending radially outwardly of thecentral portion 312 a to be freely movable with respect to thesupporting portion 311 a of the fixed case member 311. The oscillationplate 312 is adapted to be partly oscillatable along the center axiswith respect to the fixed case member 311.

The acceleration sensor 300 further comprises a piezoelectric element313 held in contact with the oscillation plate 312 to generate a voltageindicative of the acceleration when the acceleration is exerted on thesensor casing 350 to have the oscillation plate 312 partly oscillatedalong the center axis with respect to the sensor casing 350 with theperipheral portion 312 b of the oscillation plate 312 being deformed.The piezoelectric element 313 has first and second electrodes 314 a and314 b having the voltage indicative of the acceleration to outputtherethrough. The oscillation plate 312 and the piezoelectric element313 are adapted to be integrally oscillatable within a range ofeffective oscillation frequencies.

The acceleration sensor 300 further comprises at least one terminal pin317 b extending through the cover assembly 340 and terminating at theexterior of the cover assembly 340. The terminal pin 317 b is adapted tobe electrically connected with the second electrode 314 b.

In the seventh embodiment of the acceleration sensor 300, theoscillation plate 312 and the piezoelectric element 313 are integrallyoscillatable in two different modes consisting of: a {fraction (1/1)}oscillation mode where the oscillation plate 312 is irregularly deformedto have the peripheral portion 312 b oscillated with a single vector inthe oscillation direction of the oscillation plate 312 when theoscillation plate 312 is oscillated with respect to the fixed casemember 311 at a resonance frequency f₀ (see FIG. 31A); and a ½oscillation mode where the oscillation plate 312 is irregularly deformedto have two different half parts of the peripheral portion 312 boscillated with their respective different vectors opposite to eachother in the oscillation direction of the oscillation plate 312 when theoscillation plate 312 is oscillated with respect to the fixed casemember 311 at a noise frequency f₀ 1 (see FIG. 31B), and the resonancefrequency f₀ and the noise frequency f₀ 1 are out of the range ofeffective oscillation frequencies. The {fraction (1/1)} oscillation modeand ½ oscillation mode are hereinlater referred to as “the firstoscillation mode” and “the second oscillation mode”, respectively.

As described hereinbefore, the oscillation of the oscillation plate 312in the first or second oscillation mode introduces spurious of theacceleration sensor 300, which causes errors in detecting anacceleration. It is therefore required that the resonance frequency f₀and the noise frequency f₀ 1 are out of the range of effectiveoscillation frequencies actually used for detecting an acceleration inorder to improve the performance of the acceleration sensor.

The dimensions of the acceleration sensor 300 will be describedhereinlater.

As shown in FIG. 16, the supporting portion 311 a of the fixed casemember 311 of the acceleration sensor 300 has a cylindrical section 311i and a forward tapered section 311 j integrally formed with thecylindrical section 311 i and in the form of a truncated cone shape. Theforward tapered section 311 j has a top surface 311 k securely held incontact with the second surface of the oscillation plate 312 and has anouter end edge 311 l in axially alignment with the outer peripheral end312 c of the peripheral portion 312 b of the oscillation plate 312. Theouter end edge 311 l has a diameter φC₁ (mm), and the oscillation plate312 has an effective oscillation radius R₁ (mm) measured between theinner end 312 d and outer end 312 c of the peripheral portion 312 b ofthe oscillation plate 312.

The oscillation plate 312 and the piezoelectric element 313 are adaptedto be integrally oscillatable in the first and second oscillation modeswith φC₁ (mm)/R₁ (mm) and f₀ 1 /f ₀ given in the following equations:

φC ₁ (mm)/R ₁ (mm)≧0.92,  equation (2)

and

f ₀ 1 /f ₀≧0.52  equation (3)

The following description will be directed to how to determine thedimensions of the constitutional elements and parts forming part of theseventh embodiment of the acceleration sensor 300 in order to have theresonance frequency f₀ and the noise frequency f₀ 1 out of the range ofeffective oscillation frequencies actually used for detecting anacceleration.

Experiments were carried out to check the occurrence of noise, i.e.,spurious with respect to the dimensions of the oscillation plate 312 andthe piezoelectric element 313 changed under the condition that thethickness t₁ of the oscillation plate 312 and the thickness t₂ of thepiezoelectric element 313 are 2 (mm). The occurrence of spurious due tothe ½ oscillation mode is checked at a frequency of 15 kHz, which is anupper limit of the range of effective oscillation frequencies actuallyused for detecting an acceleration. The dimensions of the oscillationplate 312 and the piezoelectric element 313 changed are the outerdiameter φA₁, of the oscillation plate 312, the outer diameter φA₂ ofthe piezoelectric element 313, and the diameters φC₁ of the supportingportion 311 a.

FIG. 17 shows a table showing the result of the experiments. The legendsappearing with “◯” (pass) and “X” (fail) in the table are respectivelyintended to mean that spurious was eliminated or not, respectively.

As shown in FIG. 17, the occurrence of the spurious due to the ½oscillation mode is not recognized under a condition that φC₁ (mm)/R₁(mm) is equal to or greater than 0.92, and f₀ 1/f₀ is equal to orgreater than 0.52.

This result from the fact that the diameter φC₁ of the supportingportion 311 a is enlarged, thereby making it possible for the supportingportion 311 a to steadily support the oscillation plate 312 and thepiezoelectric element 313 to prevent the oscillation plate 312 and thepiezoelectric element 313 from oscillating in the second oscillationmode, i.e., the ½ oscillation mode at a frequency in the range ofeffective oscillation frequencies actually used for detecting anacceleration. This means that the noise frequency f₀ 1 can be moved outof the range of effective oscillation frequencies actually used fordetecting an acceleration.

Another experiments were carried out to measure the frequencycharacteristics with the diameter φC₁ of the supporting portion 311 aspecified. FIG. 18 shows the result of the experiments performed withφC₁ of 4.2 (mm), FIG. 19 shows the result of the experiments performedwith φC₁ of 5.7 (mm), FIG. 20 shows the result of the experimentsperformed with φC₁ of 6.3 (mm), and FIG. 21 shows the result of theexperiments performed with φC₁ of 7.3 (mm). As seen from FIGS. 20 and21, spurious was eliminated with φC₁/R₁ of 0.92 and φC₁/R₁ of 1.15. Thisleads to the fact that the performance of the acceleration sensor 300 isimproved.

In the acceleration sensor 300, the oscillation plate 312 and thepiezoelectric element 313 are mounted on the supporting portion 311 a ofthe fixed case member 311 with the center axes of the oscillation plate312 and the piezoelectric element 313 held in axial alignment with thecenter axis of the supporting portion 311 a of the fixed case member 311so as to detect an acceleration. Unlike the third conventionalacceleration sensor 120 shown in FIG. 29, the acceleration sensor 300 isnot required to have a weight and a fastening screw. The accelerationsensor 300 is therefore of high performance and simple in construction,thereby making it possible to automatically assemble the accelerationsensor 300, and reduce the production cost of the acceleration sensor300.

According to the present invention, the resonance frequency f₀ of theoscillation plate involving the piezoelectric element can be maintainedin a range of around 20 kHz or greater to obtain an optimizedcombination of the sensitivity V₀ and the resonance frequency f₀,thereby making it possible to enhance the performance of theacceleration sensor.

In the acceleration sensor 300, the fixed case member 311 and theoscillation plate 312 are each made of an electroconductive material toensure that the first electrode 314 a is electrically connected with theoscillation plate 312 and the fixed case member 311, thereby making itpossible for the fixed case member 311 to serve as a ground.

The acceleration sensor 300, furthermore, comprises a fixed case member311 having a screw portion 311 e to be screwed to an exterior object,which is to receive the acceleration to ensure that the oscillationplate 312 is oscillated with respect to the fixed case member 311 whenthe exterior object is oscillated.

The oscillation plate 312 of the acceleration sensor 300 is adapted tobe oscillatable with the resonance frequency f₀ of 20 kHz or greater,and the range of effective oscillation frequencies between 0 and 15 kHz,thereby making it possible to have the resonance frequency f₀ out of therange of effective oscillation frequencies actually used for detectingan acceleration. As will be understood from the foregoing description,the acceleration sensor 300 thus constructed can enhance both theresonance frequency f₀ and the sensitivity V₀, thereby making itpossible to improve the performance of the acceleration sensor 300.

The previously mentioned third to seventh embodiments of theacceleration sensor 300 has various modifications. Any one of the thirdto seventh embodiments of the acceleration sensor 300 can be replaced byone modification in order to attain the above objects of the presentinvention.

The modification of third to seventh embodiments of the accelerationsensor 300 is shown in FIG. 22. In the modification of the accelerationsensor 300 according to the present invention, for instance, the coverassembly 340 comprises a metal base member 315 made of anelectroconductive material and a cover member 316 made of an insulatingmaterial. The metal base member 315 has a peripheral end portion 315 bsecured to part 311 d of the fixed case member 311 with the closed space360 defined by the metal base member 315 and the fixed case member 311.The cover member 316 is mounted on the metal base member 315, and theterminal pin 317 b extends through the metal base member 315 and thecover member 316 to be electrically connected with the second electrode314 b of the piezoelectric element 313.

The modification of the acceleration sensor 300 according to the presentinvention comprises a printed board 319 retained by the cover assembly340 to have the second electrode 314 b of the piezoelectric element 313and the terminal pin 317 b connected with each other. The metal basemember 315 of the acceleration sensor 300 is formed with a central hole365, and the terminal pin 317 b has a contacting rod portion 325projected from the printed board 319 and extends through the centralhole 365 of the metal base member 315 to project into the closed space360 in electrical connection with the second electrode 314 b of thepiezoelectric element 313. The metal base member 315 may have aperipheral end portion 315 b welded to part 311 d of the fixed casemember 311.

In the modification of the acceleration sensor 300, the fixed casemember 311 has a large diameter portion 311 b, a small diameter portion311 c, and an annular ledge portion 311 d having the large and smalldiameter portions 311 b, 311 c integrally formed with each other to havethe peripheral end portion 315 b of the metal case member 315 firmlymounted thereon and welded thereto. The small diameter portion 311 c hasan open peripheral edge 311 n inwardly bent to be firmly engaged withthe second surface of the cover member 316. The cylindrical side portion311 h of the fixed case member 311 is larger in diameter than the covermember 316 to form an annular gap 301 between the inner surface of thesmall diameter portion 311 c of the fixed case member 311 and the outerperipheral end surface of the cover member 316, and which furthercomprises a resilient ring 318 disposed in the annular gap 301 andrested on the metal base member 315 to have the closed space 360hermetically sealed. The resilient ring 318 may be made of an O-ring.The metal base member 315 may have a peripheral end portion 315 b weldedto part of the fixed case member 311.

In the modification of the acceleration sensor 300, the fixed casemember 315 and the oscillation plate 312 are each made of anelectroconductive material to ensure that the first electrode 314 a iselectrically connected with the oscillation plate 312 and the fixed casemember 311, which serves as a ground. In the modification of theacceleration sensor 300 thus constructed, the metal base member 315, thecover member 316, and the fixed case member 311 can be securely mountedby way of an automatic assembly device, makes it easy for themodification of the acceleration sensor 300 to be automaticallyassembled. This leads to the fact that the modification of theacceleration sensor 300 are of high performance and appropriate forautomatic production at a low cost.

While there has been described about the third to seventh embodimentsand their modifications of the acceleration sensor 300 which comprisesone piezoelectric element 313 mounted on one surfaces of the oscillationplate 312, two piezoelectric elements may be mounted on the bothsurfaces of the oscillation plate 312 according to the presentinvention. The foregoing third to seventh embodiments and themodification of the acceleration sensor 300 may be replaced by eighth totwelfth embodiments of the acceleration sensor 310 comprising twopiezoelectric elements 313 a and 313 b mounted on both surfaces of theoscillation plate 312 in order to attain the above objects of thepresent invention.

The third embodiment of the acceleration sensor 300 can be modified asan eighth embodiment of the acceleration sensor 310 in a manner that thedimensions of the oscillation plate 312, the piezoelectric elements 313a and 313 b and the supporting portion 311 a of the fixed case member311 are modified. The eighth embodiment of the acceleration sensor 310as a modification of the third embodiment of the acceleration sensor 300will be described hereinafter. The constitutional elements and parts ofthe eighth embodiment of the acceleration sensor 310 same as those ofthe third embodiment of the acceleration sensor 300 are simplyrepresented by the same reference numerals as those of the thirdembodiment of the acceleration sensor 300, and will be thus omitted fromdescription for avoiding tedious repetition.

Referring to FIGS. 23 and 24 of the drawings, there is shown an eighthpreferred embodiment of an acceleration sensor 310 according to thepresent invention.

The acceleration sensor 310 is shown in FIG. 23 as further comprising afirst piezoelectric element 313 a having flat surfaces and provided onthe first flat surface of the oscillation plate 312 to generate avoltage indicative of the acceleration when the acceleration is exertedon the sensor casing 350 to have the oscillation plate 312 partlyoscillated along the center axis with respect to the sensor casing 350with the peripheral portion 312 b of the oscillation plate 312 beingdeformed.

The eighth embodiment of the acceleration sensor 310 further comprises asecond piezoelectric element 313 b having flat surfaces and provided onthe second flat surface of the oscillation plate 312 to generate avoltage indicative of the acceleration when the acceleration is exertedon the sensor casing 350 to have the oscillation plate 312 partlyoscillated along the center axis with respect to the sensor casing 350with the peripheral portion 312 b of the oscillation plate 312 beingdeformed. The first and second piezoelectric elements 313 a and 313 beach has a pair of electrodes, i.e., first, second, third and fourthelectrodes 314 a, 314 b, 314 c, 314 d having the voltage indicative ofthe acceleration to output therethrough. The oscillation plate 312 andthe first and second piezoelectric elements 313 a, 313 b are adapted tobe integrally oscillatable within a range of effective oscillationfrequencies. The first, second, third and fourth electrodes 314 a, 314b, 314 c, 314 d are adapted to allow the voltage indicative of theacceleration to output therethrough.

In the acceleration sensor 310, the first piezoelectric element 313 ahas a third electrode 314 c provided on the second surface of the firstpiezoelectric element 313 a, and second piezoelectric element 313 b hasa fourth electrode 314 d provided on the second surface of the secondpiezoelectric element 313. The fixed case member 311 and the oscillationplate 312 are each made of an electroconductive material to ensure thatthe third electrode 314 c of first piezoelectric element 313 a and thefourth electrode 314 d of the second piezoelectric element 313 b areelectrically connected with the oscillation plate 312 and the fixed casemember 311, thereby enabling the fixed case member 311 to serve as aground.

In the eighth embodiment of the acceleration sensor 310, the oscillationplate 312 has a thickness t₁ (mm), and an effective oscillation radiusR₁ (mm) measured between the inner end 312 d and outer end 312 c of theperipheral portion 312 b of the oscillation plate 312 as shown in FIG.24. The ratio of the effective oscillation radius R₁ (mm) to thethickness t₁ (mm) of the oscillation plate may be maintained within afluctuation range of 3.3 plus minus 1.5, i.e., 3.3±1.5. The ground forthe numeral setting is the same as that of the third embodiment of theacceleration sensor 300.

The acceleration sensor 310 thus constructed can enhance both theresonance frequency f₀ and the sensitivity V₀, thereby making itpossible to improve the performance of the acceleration sensor 310.

The foregoing fourth embodiment of the acceleration sensor 300 can bereplaced by a ninth embodiment of the acceleration sensor 310 comprisingtwo piezoelectric elements 313 a and 313 b mounted on both surfaces ofthe oscillation plate 312 in order to attain the above objects of thepresent invention.

The fourth embodiment of the acceleration sensor 300 can be modified asa ninth embodiment of the acceleration sensor 310 in a manner that thatthe dimensions of the oscillation plate 312, the piezoelectric elements313 a and 313 b and the supporting portion 311 a of the fixed casemember 311 are modified. The ninth embodiment of the acceleration sensor310 as a modification of the fourth embodiment of the accelerationsensor 300 will be described hereinafter. The constitutional elementsand parts of the ninth embodiment of the acceleration sensor 310 same asthose of the acceleration sensor 300 are simply represented by the samereference numerals as those of the third embodiment of the accelerationsensor 300, and will be thus omitted from description for avoidingtedious repetition.

Referring to FIGS. 23 and 24 of the drawings, there is shown a ninthembodiment of the acceleration sensor 310 according to the presentinvention. In the ninth embodiment of the acceleration sensor 310, thepiezoelectric elements 313 a and 313 b has radially outer ends 313 c and313 c′ as shown in FIG. 24. The peripheral portion 312 b of theoscillation plate 312 has a radially inner end 312 d. The length betweenthe radially outer end 313 c of the piezoelectric elements 313 a and theradially inner end 312 d of the oscillation plate 312 and the lengthbetween the radially outer end 313 c′ of the piezoelectric elements 313b and the radially inner end 312 d of the oscillation plate 312 are R₂(mm). R₂ (mm) is equal to 0.5 plus minus 0.25, multiplied by R₁ (mm),i.e., (0.5±0.25) R₁. The ground for the numeral setting is the same asthat of the fourth embodiment of the acceleration sensor 300.

The ninth embodiment of the acceleration sensor 310 thus constructed canenhance both the resonance frequency f₀ and the sensitivity V₀, therebymaking it possible to improve the performance of the acceleration sensor310.

The foregoing fifth embodiment of the acceleration sensor 300 can bereplaced by a tenth embodiment of the acceleration sensor 310 comprisingtwo piezoelectric elements 313 a and 313 b mounted on both surfaces ofthe oscillation plate 312 in order to attain the above objects of thepresent invention.

The fifth embodiment of the acceleration sensor 300 can be modified as atenth embodiment of the acceleration sensor 310 in a manner that thatthe dimensions of the oscillation plate 312, the piezoelectric elements313 a and 313 b and the supporting portion 311 a of the fixed casemember 311 are modified. The tenth embodiment of the acceleration sensor310 as a modification of the fifth embodiment of the acceleration sensor300 will be described hereinafter. The constitutional elements and partsof the tenth embodiment of the acceleration sensor 310 same as those ofthe third embodiment of the acceleration sensor 300 are simplyrepresented by the same reference numerals as those of the thirdembodiment of the acceleration sensor 300, and will be thus omitted fromdescription for avoiding tedious repetition.

Referring to FIGS. 23 and 24 of the drawings, there is shown a tenthembodiment of the acceleration sensor 310 according to the presentinvention. In the tenth embodiment of the acceleration sensor 310, thesupporting portion 311 a of the fixed case member 311 has a cylindricalsection 311 i and a forward tapered section 311 j integrally formed withthe cylindrical section 311 i and in the form of a truncated cone shapeas shown in FIG. 24. The forward tapered section 311 j has a top surfacesecurely held in contact with the second surface of the oscillationplate 312 and has an outer end edge 311 l in axially alignment with theouter peripheral end 312 c of the peripheral portion 312 b of theoscillation plate 312. The outer end edge 311 l has a diameter φC₁ (mm)and the piezoelectric elements 313 a, 313 b is in the form of an annularshape to have an inner diameter φB₂ (mm). The inner diameter φB₂ (mm) ofthe annular shape of the piezoelectric element 313 a, 313 b isapproximately equal to or greater than the diameter φC₁ (mm) of theouter end edge 311 l of the top surface of the supporting portion 311 a.The ground for the numeral setting is the same as that of the fifthembodiment of the acceleration sensor 300.

The tenth embodiment of the acceleration sensor 310 thus constructed canenhance both the resonance frequency f₀ and the sensitivity V₀, therebymaking it possible to improve the performance of the acceleration sensor310.

The foregoing sixth embodiment of the acceleration sensor 300 can bereplaced by an eleventh embodiment of the acceleration sensor 310comprising two piezoelectric elements 313 a and 313 b mounted on bothsurfaces of the oscillation plate 312 in order to attain the aboveobjects of the present invention.

The sixth embodiment of the acceleration sensor 300 can be modified asan eleventh embodiment of the acceleration sensor 310 in a manner thatthat the dimensions of the oscillation plate 312, the piezoelectricelements 313 a and 313 b and the supporting portion 311 a of the fixedcase member 311 are modified. The eleventh embodiment of theacceleration sensor 310 as a modification of the sixth embodiment of theacceleration sensor 300 will be described hereinafter. Theconstitutional elements and parts of the eleventh embodiment of theacceleration sensor 310 same as those of the third embodiment of theacceleration sensor 300 are simply represented by the same referencenumerals as those of the third embodiment of the acceleration sensor300, and will be thus omitted from description for avoiding tediousrepetition.

Referring to FIGS. 23 and 24 of the drawings, there is shown an eleventhembodiment of the acceleration sensor 310 according to the presentinvention. In the eleventh embodiment of the acceleration sensor 310,the piezoelectric elements 313 a and 313 b have thickness t₂ and t₂′,respectively as shown in FIG. 24. The thickness t₁ (mm) of theoscillation plate 312 is approximately equal to the thickness t₂ (mm)and t₂′ (mm) of the piezoelectric element 313 a and the piezoelectricelement 313 b, or the ratio of the thickness t₁ (mm) of the oscillationplate 312 with respect to the thickness t₂ of the piezoelectric element313 a and the ratio of the thickness t₁ (mm) of the oscillation plate312 with respect to the thickness t₂′ of the piezoelectric element 313 bare within a fluctuation range of 0.5 to 3. The ground for the numeralsetting is the same as that of the sixth embodiment of the accelerationsensor 300.

The eleventh embodiment of the acceleration sensor 310 thus constructedcan enhance both the resonance frequency f₀ and the sensitivity V₀,thereby making it possible to improve the performance of theacceleration sensor 310.

The foregoing seventh embodiment of the acceleration sensor 300 can bereplaced by a twelfth embodiment of the acceleration sensor 310comprising two piezoelectric elements 313 a and 313 b mounted on bothsurfaces of the oscillation plate 312 in order to attain the aboveobjects of the present invention.

The seventh embodiment of the acceleration sensor 300 can be modified asa twelfth embodiment of the acceleration sensor 310 in a manner thatthat the dimensions of the oscillation plate 312, the piezoelectricelements 313 a and 313 b and the supporting portion 311 a of the fixedcase member 311 are modified. The twelfth embodiment of the accelerationsensor 310 as a modification of the seventh embodiment of theacceleration sensor 300 will be described hereinafter. Theconstitutional elements and parts of the twelfth embodiment of theacceleration sensor 310 same as those of the third acceleration sensor300 are simply represented by the same reference numerals as those ofthe third embodiment of the acceleration sensor 300, and will be thusomitted from description for avoiding tedious repetition.

Referring to FIGS. 23 and 24 of the drawings, there is shown an twelfthembodiment of the acceleration sensor 310 according to the presentinvention. In the twelfth acceleration sensor 310, the oscillation plate312 and the piezoelectric elements 313 a and 313 b are integrallyoscillatable in two different modes consisting of: a {fraction (1/1)}oscillation mode where the oscillation plate 312 is irregularly deformedto have the peripheral portion 312 b oscillated with a single vector inthe oscillation direction of the oscillation plate 312 when theoscillation plate 312 is oscillated with respect to the fixed casemember 311 at a resonance frequency f₀ (see FIG. 31A); and a ½oscillation mode where the oscillation plate 312 is irregularly deformedto have two different half parts of the peripheral portion 312 boscillated with their respective different vectors opposite to eachother in the oscillation direction of the oscillation plate 312 when theoscillation plate 312 is oscillated with respect to the fixed casemember 311 at a noise frequency f₀ 1 (see FIG. 31B), and the resonancefrequency f₀ and the noise frequency f₀ 1 are out of the range ofeffective oscillation frequencies. The {fraction (1/1)} oscillation modeand ½ oscillation mode are hereinlater referred to as “the firstoscillation mode” and “The second oscillation mode”, respectively. Theground for the definition is the same as that of the seventh embodimentof the acceleration sensor 300.

The acceleration sensor 310 thus constructed can enhance both theresonance frequency f₀ and the sensitivity V₀, thereby making itpossible to improve the performance of the acceleration sensor 310.

The following description will be directed to how to determine thedimensions of the constitution elements and parts forming port of theseventh embodiment of the acceleration sensor 300.

As shown in FIG. 24, the supporting portion 311 a of the fixed casemember 311 of the acceleration sensor 300 has a cylindrical section 311i and a forward tapered section 311 j integrally formed with thecylindrical section 311 i and in the form of a truncated cone shape. Theforward tapered section 311 j has a top surface 311 k securely held incontact with the second surface of the oscillation plate 312 and has anouter end edge 311 l in axially alignment with the outer peripheral end312 c of the peripheral portion 312 b of the oscillation plate 312. Theouter end edge 311 l has a diameter φC₁ (mm), and the oscillation plate312 has an effective oscillation radius R₁ (mm) measured between theinner end 312 d and outer end 312 c of the peripheral portion 312 b ofthe oscillation plate 312.

The oscillation plate 312 and the piezoelectric elements 313 a and 313 bare adapted to be integrally oscillatable in the first and secondoscillation modes with φC₁, (mm)/R₁ (mm) and f₀ 1 /f₀ given in thefollowing equations:

φC ₁ (mm)/R ₁ (mm)≧0.92,  equation (2)

and

 f ₀1/f ₀≧0.52  equation (3)

The ground for the definition is the same as that of the seventhembodiment of the acceleration sensor 300.

The acceleration sensor 310 thus constructed makes it possible for thesupporting portion 311 a to steadily support steadily the oscillationplate 312 and the piezoelectric elements 313 a and 313 b to prevent theoscillation plate 312 and the piezoelectric elements 313 a and 313 bfrom oscillating in the second oscillation mode, i.e., the ½ oscillationmode at a frequency in the range of effective oscillation frequenciesactually used for detecting an acceleration. This means that the noisefrequency f₀ 1 can be moved out of the range of effective oscillationfrequencies actually used for detecting an acceleration. This means thatthe acceleration sensor 310 thus constructed can enhance both theresonance frequency f₀ and the sensitivity V₀, thereby making itpossible to improve the performance of the acceleration sensor 310.

As will be seen from the foregoing description, the acceleration sensor310 according to the present invention is of high performance and can beproduced at a low cast. The reason is due to the fact that the fixedcase member 311 and the cover assembly 340 define a closed space 360accommodating the oscillation plate 312 and the piezoelectric elements313 a and 313 b received therein, and the oscillation plate 312 and thepiezoelectric elements 313 a and 313 b are oscillatably supported by thesupporting portion 311 a formed on the central bottom portion of thefixed case member 311, and the first and second electrodes 314 a and 314b of the piezoelectric elements 313 a and 313 b are electricallyconnected with the exterior connector through the terminal pin 317 b,thereby reducing the number of the constitutional elements and parts andfacilitating the automatic assembly. Furthermore, the oscillation plate312 and the piezoelectric elements 313 a and 313 b are adapted to beintegrally oscillatable in the first and second oscillation modes withφC₁ (mm)/R₁ (mm) and f₀ 1/f₀ given in the following equations:

φC ₁ (mm)/R ₁ (mm)≧0.92,

and

f ₀ 1/f ₀≧0.5,

thereby making it possible for the noise frequency f₀ 1 to be moved outof the range of effective oscillation frequencies actually used fordetecting an acceleration

It will be apparent to those skilled in the art and it is contemplatedthat variations and/or changes in the embodiments illustrated anddescribed herein may be without departure from the present invention.Accordingly, it is intended that the foregoing description isillustrative only, not limiting, and that the true spirit and scope ofthe present invention will be determined by the appended claims.

What is claimed is:
 1. An acceleration sensor for detecting anacceleration caused by an object oscillated in an oscillation direction,comprising: a sensor casing having a center axis and to be positioned incoaxial alignment with said oscillation direction to receive saidacceleration, said sensor casing including a cylindrical fixed casemember and a cover assembly provided on said fixed case member, saidfixed case member having a circular bottom portion having a firstcircular inner surface, a cylindrical side portion integrally formedwith said bottom portion, and a supporting portion projecting from saidbottom portion, said cover assembly having a circular cover portionhaving a second circular inner surface, and a cylindrical side portionintegrally formed with said cover portion, said side portion of saidsensor casing partly having a third cylindrical inner surface connectedat one end with said first inner surface of said bottom portion, saidside portion of said cover assembly partly having said third cylindricalinner surface connected at the other one end with said second innersurface of said cover portion, said first inner surface of said bottomportion of said fixed case member, said second inner surface of saidcover portion of said cover assembly, said third inner surface of saidside portion of said fixed case member, and said third inner surface ofsaid side portion of said cover assembly collectively defining acylindrical closed space; an oscillation plate accommodated in saidclosed space of said sensor casing and having a central portionsupported by said supporting portion of said fixed case member of saidsensor casing, and a peripheral portion integrally formed with saidcentral portion and extending radially outwardly of said central portionto be freely movable with respect to said supporting portion of saidfixed case member of said sensor casing, said oscillation plate having afirst surface opposing to and spaced apart from said cover assembly anda second surface opposing to and spaced apart from said bottom portionof said fixed case member, said oscillation plate being partlyoscillatable along said center axis with respect to said fixed casemember, and a piezoelectric element held in contact with saidoscillation plate to generate a voltage indicative of said accelerationwhen said acceleration is exerted on said sensor casing to have saidoscillation plate partly oscillated along said center axis with respectto said sensor casing with said peripheral portion of said oscillationplate being deformed, said piezoelectric element having first and secondelectrodes mounted thereon, said first electrode disposed between saidpiezoelectric element and said oscillation plate, said second electrodeopposing said cover assembly, said first and second electrodes allowingsaid voltage indicative of said acceleration to output therethrough,said oscillation plate having a thickness t₁ (mm), and an effectiveoscillation radius R₁ (mm) measured between the inner and outer ends ofsaid peripheral portion of said oscillation plate, wherein the ratio ofsaid effective oscillation radius R₁ (mm) to said thickness t₁ (mm) ofthe oscillation plate is maintained within a fluctuation range given bythe equation as follows. R ₁ /t 1=3.3±1.5
 2. An acceleration sensor fordetecting an acceleration as set forth in claim 1 in which saidpiezoelectric element has a radially outer end, said peripheral portionof said oscillation plate has a radially inner end, the length betweensaid radially outer end of said piezoelectric element and said radiallyinner end of said oscillation plate being R₂ (mm), wherein R₂ (mm) isgiven by the equation as follows. R ₂=(0.5±0.25)R ₁.
 3. An accelerationsensor for detecting an acceleration as set forth in claim 1 or claim 2,in which said supporting portion of said fixed case member has acylindrical section and a forward tapered section integrally formed withsaid cylindrical section and in the form of a truncated cone shape, saidforward tapered section having a top surface securely held in contactwith said second surface of said oscillation plate and having an outerend edge in axially alignment with said outer peripheral end of saidperipheral portion of said oscillation plate, said outer end edge havinga diameter φC₁ (mm) and said piezoelectric element being in the form ofan annular shape to have an inner diameter φB₂ (mm), whereby said innerdiameter φB₂ (mm) of said annular shape of said piezoelectric element isapproximately equal to or greater than said diameter φC₁ (mm) of saidouter end edge of said top surface of said supporting portion.
 4. Anacceleration sensor for detecting an acceleration as set forth in claim1, in which said piezoelectric element has a thickness t₂, whereby thethickness t₁ (mm) of said oscillation plate is approximately equal tothe thickness t₂ (mm) of said piezoelectric element, or the ratio of thethickness t₁ (mm) of said oscillation plate to the thickness t₂ of saidpiezoelectric element is within a fluctuation range of 0.5 to
 3. 5. Anacceleration sensor for detecting an acceleration caused by an objectoscillated in an oscillation direction, comprising: a sensor casinghaving a center axis and to be positioned in coaxial alignment with saidoscillation direction to receive said acceleration, said sensor casingincluding a cylindrical fixed case member and a cover assembly providedon said fixed case member, said fixed case member having a circularbottom portion having a first circular inner surface, a cylindrical sideportion integrally formed with said bottom portion, and a supportingportion projecting from said bottom portion, said cover assembly havinga circular cover portion having a second circular inner surface, and acylindrical side portion integrally formed with said cover portion, saidside portion of said sensor casing partly having a third cylindricalinner surface connected at one end with said first inner surface of saidbottom portion, said side portion of said cover assembly partly havingsaid third cylindrical inner surface connected at the other one end withsaid second inner surface of said cover portion, said first innersurface of said bottom portion of said fixed case member, said secondinner surface of said cover portion of said cover assembly, said thirdinner surface of said side portion of said fixed case member, and saidthird inner surface of said side portion of said cover assemblycollectively defining a cylindrical closed space; an oscillation plateaccommodated in said closed space of said sensor casing and having acentral portion supported by said supporting portion of said fixed casemember of said sensor casing, and a peripheral portion integrally formedwith said central portion and extending radially outwardly of saidcentral portion to be freely movable with respect to said supportingportion of said fixed case member of said sensor casing, saidoscillation plate having a first surface opposing to and spaced apartfrom said cover assembly and a second surface opposing to and spacedapart from said bottom portion of said fixed case member, saidoscillation plate being partly oscillatable along said center axis withrespect to said fixed case member; a first piezoelectric element havingflat surfaces and provided on said first flat surface of saidoscillation plate to generate a voltage indicative of said accelerationwhen said acceleration is exerted on said sensor casing to have saidoscillation plate partly oscillated along said center axis with respectto said sensor casing with said peripheral portion of said oscillationplate being deformed; a second piezoelectric element having flatsurfaces and provided on said second flat surface of said oscillationplate to generate a voltage indicative of said acceleration when saidacceleration is exerted on said sensor casing to have said oscillationplate partly oscillated along said center axis with respect to saidsensor casing with said peripheral portion of said oscillation platebeing deformed, said first and second piezoelectric elements each havinga pair of electrodes having said voltage indicative of said accelerationto output therethrough, and said oscillation plate and said first andsecond piezoelectric elements being integrally oscillatable within arange of effective oscillation frequencies, said electrodes allowingsaid voltage indicative of said acceleration to output therethrough,said oscillation plate having a thickness t₁ (mm), and an effectiveoscillation radius R₁ (mm) measured between the inner and outer ends ofsaid peripheral portion of said oscillation plate, wherein the ratio ofsaid effective oscillation radius R₁ (mm) to said thickness t₁ (mm) ofthe oscillation plate is maintained within a fluctuation range given bythe equation as follows. R ₁ /t 1=3.3±1.5
 6. An acceleration sensor fordetecting an acceleration as set forth in claim 5 in which saidpiezoelectric element has a radially outer end, said peripheral portionof said oscillation plate has a radially inner end, the length betweensaid radially outer end of said piezoelectric element and said radiallyinner end of said oscillation plate being R₂ (mm), wherein R₂ (mm) isgiven by the equation as follows. R ₂=(0.5±0.25)R ₁.
 7. An accelerationsensor for detecting an acceleration as set forth in claim 5 or claim 6,in which said supporting portion of said fixed case member has acylindrical section and a forward tapered section integrally formed withsaid cylindrical section and in the form of a truncated cone shape, saidforward tapered section having a top surface securely held in contactwith said second surface of said oscillation plate and having an outerend edge in axially alignment with said outer peripheral end of saidperipheral portion of said oscillation plate, said outer end edge havinga diameter φC₁ (mm) and said piezoelectric element being in the form ofan annular shape to have an inner diameter φB₂ (mm), whereby said innerdiameter φB₂ (mm) of said annular shape of said piezoelectric element isapproximately equal to or greater than said diameter φC₁ (mm) of saidouter end edge of said top surface of said supporting portion.
 8. Anacceleration sensor for detecting an acceleration as set forth in claim5, in which said piezoelectric element has a thickness t₂, whereby thethickness t₁ (mm) of said oscillation plate is approximately equal tothe thickness t₂ (mm) of said piezoelectric element, or the ratio of thethickness t₁ (mm) of said oscillation plate to the thickness t₂ of saidpiezoelectric element is within a fluctuation range of 0.5 to
 3. 9. Anacceleration sensor for detecting an acceleration as set forth in anyone of claims 1 and 5, in which said cover assembly comprises a metalbase member made of an electroconductive material and a cover membermade of an insulating material, said metal base member having aperipheral end portion secured to part of said fixed case member withsaid closed space defined by said metal base member and said fixed casemember, said cover member being mounted on said metal base member, andsaid terminal pin extending through said metal base member and saidcover member and electrically connected with said second electrode ofsaid piezoelectric element.
 10. An acceleration sensor for detecting anacceleration as set forth in claim 9, in which said metal base member isformed with a central hole, and said terminal pin having a contactingrod portion extending through said central hole of said metal basemember to project into said closed space in electrical connection withsaid second electrode of said piezoelectric element.
 11. An accelerationsensor for detecting an acceleration as set forth in claim 9, in whichsaid cylindrical side portion is larger in diameter than said covermember to form an annular gap between the inner surface of said smalldiameter portion of said fixed case member and said outer peripheral endsurface of said cover member, and which further comprises a resilientring disposed in said annular gap and rested on said metal base memberto have said closed space hermetically sealed.
 12. An accelerationsensor for detecting an acceleration as set forth in claim 11, in whichsaid resilient ring is made of an O-ring.
 13. An acceleration sensor fordetecting an acceleration as set forth in claim 9, in which said metalbase member having a peripheral end portion welded to part of said fixedcase member.
 14. An acceleration sensor for detecting an acceleration asset forth in claim 9, in which said fixed case member has a largediameter portion, a small diameter portion, and an annular ledge portionhaving said large and small diameter portions integrally formed witheach other to have said peripheral end portion of said metal case memberfirmly mounted thereon and welded thereto, said small diameter portionhaving an open peripheral edge inwardly bent to be firmly engaged withsaid second surface of said cover member.
 15. An acceleration sensor fordetecting an acceleration as set forth in claim 14, in which said metalbase member having a peripheral end portion secured to part of saidfixed case member with said closed space defined by said metal basemember and said fixed case member, said cover member being mounted onsaid metal base member, and said terminal pin extending through saidmetal base member and said cover member and electrically connected withsaid one of said electrodes of said piezoelectric element.
 16. Anacceleration sensor (300, 310) for detecting an accelerator as set forthin claim 14, in which said peripheral end portion (315 b) of said metalbase member (315) is welded to said part of said fixed case member(311).
 17. An acceleration sensor for detecting an acceleration as setforth in any one of claims 1 and 5, in which said fixed case member hasa screw portion to be screwed to an exterior object which is to receivesaid acceleration.
 18. An acceleration sensor for detecting anacceleration as set forth in any one of claims 1 and 5, in which saidresonance frequency f₀ is 20 kHz or greater.
 19. An acceleration sensorfor detecting an acceleration as set forth in any one of claims 1 and 5,in which said range of effective oscillation frequencies is between 0and 15 kHz.